Network Working Group Y. Rekhter
Request for Comments: DRAFT T.J. Watson Research Center, IBM Corp.
T.Li
cisco Systems
E. Gerich
Merit Network Inc.
September 1992
Guidelines for IP Address Allocation
Status of this Memo
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1 Introduction
This paper provides guidelines and a plan for allocating IP addresses
in the Internet. These guidelines and the plan are intended to play
an important role in steering the Internet towards the Address
Assignment and Aggregating Strategy outlined in [1].
2 Scope
The global internet can be modeled as a collection of hosts
interconnected via transmission and switching facilities. Control
over the collection of hosts and the transmission and switching
facilities that compose the networking resources of the global
internet is not homogeneous, but is distributed among multiple
administrative authorities. Resources under control of a single
administration form a domain. For the rest of this paper, `domain'
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and `routing domain' will be used interchangeably.
Domains that share their resources with other domains are called
service providers (or just providers). Domains that utilize other
domain's resources are called service subscribers (or just
subscribers). A given domain may act as a provider and a subscriber
simultaneously.
There are two aspects of interest when discussing IP address
allocation within the Internet. The first is the set of
administrative requirements for obtaining and allocating IP
addresses; the second is the technical aspect of such assignments,
having largely to do with routing, both within a routing domain
(intra-domain routing) and between routing domains (inter-domain
routing). This paper focuses on the technical issues.
The technical issues in IP address allocation are mainly related to
routing.
In the current Internet many routing domains (such as corporate and
campus networks) attach to transit networks (such as NSFNET
regionals) in only one or a small number of carefully controlled
access points. The former act as subscribers, while the latter act
as providers.
The guidelines provided in this paper are intended for immediate
deployment. This paper specifically does not address long-term
research issues, such as complex policy-based routing requirements.
Addressing solutions which require substantial changes or constraints
on the current topology are not considered.
The guidelines in this paper are oriented primarily toward the
large-scale division of IP address allocation in the Internet. Topics
covered include:
- Benefits of encoding some topological information in IP
addresses to reduce routing protocol overhead;
- The anticipated need for additional levels of hierarchy in
Internet addressing to support network growth;
- The recommended mapping between Internet topological entities
(i.e., service providers, and service subscribers) and IP
addressing and routing components;
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- The recommended division of IP address assignment authority
among service providers (e.g. backbones, regionals), and
service subscribers (e.g. sites);
- Qualifications for Distributed Regional Registries;
- Allocation of the IP addresses by the Internet Registry;
- Background information on administrative procedures for
registration of administrative authorities immediately below
the national level; and,
- Choice of the high-order portion of the IP addresses in leaf
routing domains that are connected to more than one service
provider (e.g. backbone or a regional network).
It is noted that there are other aspects of IP address allocation,
both technical and administrative, that are not covered in this
paper. Topics not covered or mentioned only superficially include:
- Identification of specific administrative domains in the
Internet;
- Policy or mechanisms for making registered information known to
third parties (such as the entity to which a specific IP
address or a potion of the IP address space has been
allocated);
- How a routing domain (especially a site) should organize its
internal topology or allocate portions of its IP address space;
the relationship between topology and addresses is discussed,
but the method of deciding on a particular topology or internal
addressing plan is not; and,
- Procedures for assigning the host IP addresses.
3 Background
Some background information is provided in this section that is
helpful in understanding the issues involved in IP address
allocation. A brief discussion of IP routing is provided.
IP partitions the routing problem into three parts:
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- routing exchanges between end systems and routers (ARP),
- routing exchanges between routers in the same routing domain
(interior routing), and,
- routing among routing domains (exterior routing).
4 IP Addresses and Routing
For the purposes of this paper, an IP prefix is an IP address and
some indication of the leftmost contiguous significant bits within
this address. Throughout this paper IP address prefixes will be
expressed as <IP-address IP-mask> tuples, such that a bitwise logical
AND operation on the IP-address and IP-mask components of a tuple
yields the sequence of leftmost contiguous significant bits that form
the IP address prefix. For example a tuple with the value <193.1.0.0
255.255.0.0> denotes an IP address prefix with 16 leftmost contiguous
significant bits.
When determining an administrative policy for IP address assignment,
it is important to understand the technical consequences. The
objective behind the use of hierarchical routing is to achieve some
level of routing data abstraction, or summarization, to reduce the
cpu, memory, and transmission bandwidth consumed in support of
routing.
While the notion of routing data abstraction may be applied to
various types of routing information, this paper focuses on one
particular type, namely reachability information. Reachability
information describes the set of reachable destinations. Abstraction
of reachability information dictates that IP addresses be assigned
according to topological routing structures. However, administrative
assignment falls along organizational or political boundaries. These
may not be congruent to topological boundaries and therefore the
requirements of the two may collide. It is necessary to find a
balance between these two needs.
Routing data abstraction occurs at the boundary between
hierarchically arranged topological routing structures. An element
lower in the hierarchy reports summary routing information to its
parent(s).
At routing domain boundaries, IP address information is exchanged
(statically or dynamically) with other routing domains. If IP
addresses within a routing domain are all drawn from distinct IP
address assignment authorities (allowing no abstraction), then the
boundary information consists of an enumerated list of all the IP
addresses.
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Alternatively, should the routing domain draw IP addresses for all
the hosts within the domain from a single IP address assignment
authority, boundary routing information can be summarized into the
single IP address prefix. This permits substantial data reduction
and allows better scaling (as compared to the uncoordinated
addressing discussed in the previous paragraph).
If routing domains are interconnected in a more-or-less random (i.e.,
non-hierarchical) scheme, it is quite likely that no further
abstraction of routing data can occur. Since routing domains would
have no defined hierarchical relationship, administrators would not
be able to assign IP addresses within the domains out of some common
prefix for the purpose of data abstraction. The result would be flat
inter-domain routing; all routing domains would need explicit
knowledge of all other routing domains that they route to. This can
work well in small and medium sized internets, up to a size somewhat
larger than the current Internet. However, this does not scale to
very large internets. For example, we expect growth in the future to
an Internet which has tens or hundreds of thousands of routing
domains in North America alone. This requires a greater degree of
the reachability information abstraction beyond that which can be
achieved at the `routing domain' level.
In the Internet, however, it should be possible to exploit the
existing hierarchical routing structure interconnections, as
discussed in Section 5. Thus, there is the opportunity for a group of
routing domains each to be assigned an address prefix from a shorter
prefix assigned to another routing domain whose function is to
interconnect the group of routing domains. Each member of the group
of routing domains now `owns' its (somewhat longer) prefix, from
which it assigns its addresses.
The most straightforward case of this occurs when there is a set of
routing domains which are all attached only to a single service
provider domain (e.g. regional network), and which use that provider
for all external (inter-domain) traffic. A small prefix may be
assigned to the provider, which then assigns slightly longer prefixes
(based on the provider's prefix) to each of the routing domains that
it interconnects. This allows the provider, when informing other
routing domains of the addresses that it can reach, to abbreviate the
reachability information for a large number of routing domains as a
single prefix. This approach therefore can allow a great deal of
hierarchical abbreviation of routing information, and thereby can
greatly improve the scalability of inter-domain routing.
Clearly, this approach is recursive and can be carried through
several iterations. Routing domains at any `level' in the hierarchy
may use their prefix as the basis for subsequent suballocations,
assuming that the IP addresses remain within the overall length and
structure constraints.
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At this point, we observe that the number of nodes at each lower
level of a hierarchy tends to grow exponentially. Thus the greatest
gains in the reachability information abstraction occur at the leaves
and the gains drop significantly at each higher level. Therefore, the
law of diminishing returns suggests that at some point data
abstraction ceases to produce significant benefits. Determination of
the point at which data abstraction ceases to be of benefit requires
a careful consideration of the number of routing domains that are
expected to occur at each level of the hierarchy (over a given period
of time), compared to the number of routing domains and address
prefixes that can conveniently and efficiently be handled via dynamic
inter-domain routing protocols.
4.1 Efficiency versus Decentralized Control.
There is a balance that must be sought between the requirements on IP
addresses for efficient routing and the need for decentralized
address administration. A proposal described in section 6.3 offers an
example of how these two needs might be met.
The IP address prefix <216.0.0.0 248.0.0.0> provides for
administrative decentralization. This prefix identifies the address
allocation authority for North America. The lower order part of that
prefix allows allocation of IP addresses along topological boundaries
in support of increased data abstraction. Regionals within North
America assign IP address prefixes for their clients underneath their
unique address prefixes. Within a routing domain addresses for
subnetworks and hosts are allocated from the unique IP prefix
assigned to the domain.
5 IP Address Administration and Routing in the Internet
Internet routing components---service providers (e.g. backbones,
regional networks), and service subscribers (e.g. sites or campuses-
--are arranged hierarchically for the most part. A natural mapping
from these components to IP routing components is that providers and
subscribers act as routing domains.
Alternatively, a subscriber (e.g. a site) may choose to operate as a
part of a domain formed by a service provider. We assume that some,
if not most, sites will prefer to operate as part of their provider's
routing domain. Such sites can exchange routing information with
their provider via interior routing protocol route leaking or via an
exterior routing protocol. For the purposes of this discussion, the
choice is not significant. The site is still allocated a prefix from
the provider's address space, and the provider will advertise its own
prefix into inter-domain routing.
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Given such a mapping, where should address administration and
allocation be performed to satisfy both administrative
decentralization and data abstraction? Three possibilities are
considered:
1. at some part within a routing domain,
2. at the leaf routing domain, and,
3. at the transit routing domain (TRD).
A part within a routing domain corresponds to a subnetwork. If a
domain is composed of multiple subnetworks, they are interconnected
via routers. Leaf routing domains correspond to sites, where the
primary purpose is to provide intra-domain routing services. Transit
routing domains are deployed to carry transit (i.e., inter-domain)
traffic; backbones and providers are TRDs.
The greatest burden in transmitting and operating on routing
information is at the top of the routing hierarchy, where routing
information tends to accumulate. In the Internet, for example,
providers must manage the set of network numbers for all networks
reachable through the provider. Traffic destined for other providers
is generally routed to the backbones (which act as providers as
well). The backbones, however, must be cognizant of the network
numbers for all attached providers and their associated networks.
In general, the advantage of abstracting routing information at a
given level of the routing hierarchy is greater at the higher levels
of the hierarchy. There is relatively little direct benefit to the
administration that performs the abstraction, since it must maintain
routing information individually on each attached topological routing
structure.
For example, suppose that a given site is trying to decide whether to
obtain an IP address prefix directly from the North America address
allocation authority, or from its service provider. If considering
only their own self-interest, the site itself and the attached
provider have little reason to choose one approach or the other. The
site must use one prefix or another; the source of the prefix has
little effect on routing efficiency within the site. The provider
must maintain information about each attached site in order to route,
regardless of any commonality in the prefixes of the sites.
However, there is a difference when the provider distributes routing
information to other providers (e.g. backbones or TRDs). In the
first case, the provider cannot aggregate the site's address into its
own prefix; the address must be explicitly listed in routing
exchanges, resulting in an additional burden to other providers which
must exchange and maintain this information.
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In the second case, each other provider (e.g. backbone or TRD) sees a
single address prefix for the provider, which encompasses the new
site. This avoids the exchange of additional routing information to
identify the new site's address prefix. Thus, the advantages
primarily accrue to other providers which maintain routing
information about this site and provider.
One might apply a supplier/consumer model to this problem: the higher
level (e.g., a backbone) is a supplier of routing services, while the
lower level (e.g., a TRD) is the consumer of these services. The
price charged for services is based upon the cost of providing them.
The overhead of managing a large table of addresses for routing to an
attached topological entity contributes to this cost.
The Internet, however, is not a market economy. Rather, efficient
operation is based on cooperation. The guidelines discussed below
describe reasonable ways of managing the IP address space that
benefit the entire community.
5.1 Administration of IP addresses within a domain.
If individual subnetworks take their IP addresses from a myriad of
unrelated IP allocation authorities, there will be effectively no
data abstraction beyond what is built into existing intra-domain
routing protocols. For example, assume that within a routing domain
uses three independent prefixes assigned from three different
attached providers.
This does have a negative effect on inter-domain routing,
particularly on those other domains which need to maintain routes to
this domain. There is no common prefix that can be used to represent
these IP addresses and therefore no summarization can take place at
the routing domain boundary. When addresses are advertised by this
routing domain to other routing domains, an enumerated list must be
used consisting of the three network addresses.
This situation is roughly analogous to the present dissemination of
routing information in the Internet, where each domain may have non-
contiguous network numbers assigned to it. The result of allowing
subnetworks within a routing domain to take their IP addresses from
unrelated authorities is flat routing at the A/B/C class network
level. The number of IP prefixes that leaf routing domains would
advertise is on the order of the number of attached network numbers;
the number of prefixes a provider's routing domain would advertise is
approximately the number of network numbers attached to the client
leaf routing domains; and for a backbone this would be summed across
all attached providers. Although this situation is just barely
acceptable in the current Internet, as the Internet grows this will
quickly become intractable. A greater degree of hierarchical
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information reduction is necessary to allow continued growth in the
Internet.
5.2 Administration at the Leaf Routing Domain
As mentioned previously, the greatest degree of data abstraction
comes at the lowest levels of the hierarchy. Providing each leaf
routing domain (that is, site) with a prefix from its provider's
prefix results in the biggest single increase in abstraction. From
outside the leaf routing domain, the set of all addresses reachable
in the domain can then be represented by a single prefix. Further,
all destinations reachable within the provider's prefix can be
represented by a single prefix.
For example, consider a single campus which is a leaf routing domain
which would currently require 4 different IP networks. Under the new
allocation scheme, they might instead be given a single prefix which
provides the same number of destination addresses. Further, since
the prefix is a subset of the provider's prefix, they impose no
additional burden on the higher levels of the routing hierarchy.
There is a close relationship between subnetworks and routing domains
implicit in the fact that they operate a common routing protocol and
are under the control of a single administration. The routing domain
administration subdivides the domain into subnetworks. The routing
domain represents the only path between a subnetwork and the rest of
the internetwork. It is reasonable that this relationship also extend
to include a common IP addressing authority. Thus, the subnetworks
within the leaf RD should take their IP addresses from the prefix
assigned to the leaf RD.
5.3 Administration at the Transit Routing Domain
Two kinds of transit routing domains are considered, direct providers
and indirect providers. Most of the subscribers of a direct provider
are domains that act solely as service subscribers (they carry no
transit traffic). Most of the subscribers of an indirect provider are
domains that, themselves, act as service providers. In present
terminology a backbone is an indirect provider, while a TRD is a
direct provider. Each case is discussed separately below.
5.3.1 Direct Service Providers
It is interesting to consider whether direct service providers'
routing domains should be the common authority for assigning IP
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addresses from a unique prefix to the leaf routing domains that they
serve. The benefits derived from data abstraction are greater than in
the case of leaf routing domains, and the additional degree of data
abstraction provided by this may be necessary in the short term.
As an illustration consider an example of a direct provider that
serves 100 clients. If each client takes its addresses from 4
independent address assignment authorities then the total number of
entries that are needed to handle routing to these clients is 100
clients times 4 providers = 400. If each client takes its addresses
from a single address assignment authority then the total number of
entries would be only 100. Finally, if all the clients take their
addresses from the same address assignment authority then the total
number of entries would be only 1.
We expect that in the near term the number of routing domains in the
Internet will grow to the point that it will be infeasible to route
on the basis of a flat field of routing domains. It will therefore be
essential to provide a greater degree of information abstraction.
Direct providers may assign prefixes to leaf domains, based on a
single (shorter length) address prefix assigned to the provider. For
example, following the proposal suggested in section 6.3, a direct
provider may act as an address assignment authority and routing
domain values may be assigned by the direct provider to each attached
leaf routing domain. This results in direct providers advertising to
backbones a small fraction of the number of address prefixes that
would be necessary if they enumerated the individual prefixes of the
leaf routing domains. This represents a significant savings given
the expected scale of global internetworking.
Are leaf routing domains willing to accept prefixes derived from the
direct providers? In the supplier/consumer model, the direct provider
is offering connectivity as the service, priced according to its
costs of operation. This includes the `price' of obtaining service
from one or more indirect providers (e.g. backbones). In general,
indirect providers will want to handle as few address prefixes as
possible to keep costs low. In the Internet environment, which does
not operate as a typical marketplace, leaf routing domains must be
sensitive to the resource constraints of the providers (both direct
and indirect). The efficiencies gained in routing clearly warrant the
adoption of IP address administration by the providers.
The mechanics of this scenario are straightforward. Each direct
provider is assigned a unique small set of IP address prefixes, from
which it allocates slightly longer routing domain prefixes for its
attached leaf routing domains. For example assume that NIST is a
leaf routing domain whose sole inter-domain link is via SURANet. If
SURANet is assigned an unique IP address prefix <193.1.0.0
255.255.0.0>, NIST could be assigned a unique IP prefix of <193.1.0.0
255.255.240.0>.
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5.3.2 Indirect Providers (Backbones)
There does not appear to be a strong case for direct providers to
take their address spaces from the the IP space of an indirect
provider (e.g. backbone). The benefit in routing data abstraction is
relatively small. The number of direct providers today is in the tens
and an order of magnitude increase would not cause an undue burden on
the backbones. Also, it may be expected that as time goes by there
will be increased direct interconnection of the direct providers,
leaf routing domains directly attached to the backbones, and
international links directly attached to the providers. Under these
circumstances, the distinction between direct and indirect providers
may become blurred.
An additional factor that discourages allocation of IP addresses from
a backbone prefix is that the backbones and their attached providers
are perceived as being independent. providers may take their long-
haul service from one or more backbones, or may switch backbones
should a more cost-effective service be provided elsewhere. Having IP
addresses derived from a backbone is inconsistent with the nature of
the relationship.
5.3.3 Continental aggregation
Another level of hierarchy may also be used in this addressing scheme
to further reduce the amount of routing information necessary for
inter-continental routing. Continental aggregation is useful because
continental boundaries provide natural barriers to topological
connection and administrative boundaries. Thus, it presents a
natural boundary for another level of aggregation. To make use of
this, it is necessary that each continent be assigned an addressing
authority and an appropriate subset of the address space. Providers
(both direct and indirect) within that continent would allocate their
addresses from this space. Note that there are numerous exceptions
to this, in which a service provider (either direct or indirect)
spans a continental division. These exceptions can be handled
similarly to multi-home routing domains, as discussed below.
Note that, in contrast to the case of providers, the aggregation of
continental routing information may not be done on the continent
which `owns' the prefix. The cost of inter-continental links (and
especially trans-oceanic links) is very high. If aggregation is
performed on the `near' side of the link, then routing information
about unreachable destinations within that continent can only reside
on that continent. Alternatively, if continental aggregation is done
on the `far' side of an inter-continental link, the `far' end can
perform the aggregation and inject it into continental routing. This
means that destinations which are part of the continental
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aggregation, but for which there is not a corresponding more specific
prefix can be rejected before leaving the continent on which they
originated.
For example, suppose that Europe is assigned a prefix of <193.0.0.0
255.0.0.0>, and that European routing also contains the longer
prefixes <193.1.0.0 255.255.0.0> and <193.2.0.0 255.255.0.0>. All of
the longer European prefixes may be advertised across a trans-
Atlantic link to North America. The router in North America would
then aggregate these routes, and only advertise the prefix <193.0.0.0
255.0.0.0> into North American routing. Packets which are destined
for 193.1.1.1 would traverse North American routing, but would
encounter the North American router which performed the European
aggregation. If the prefix <193.1.0.0 255.255.0.0> is unreachable,
the router would drop the packet and send an ICMP Unreachable without
using the trans-Atlantic link.
5.4 Multi-homed Routing Domains
The discussions in Section 5.3 suggest methods for allocating IP
addresses based on direct or indirect provider connectivity. This
allows a great deal of information reduction to be achieved for those
routing domains which are attached to a single TRD. In particular,
such routing domains may select their IP addresses from a space
allocated to them by the direct provider. This allows the provider,
when announcing the addresses that it can reach to other providers,
to use a single address prefix to describe a large number of IP
addresses corresponding to multiple routing domains.
However, there are additional considerations for routing domains
which are attached to multiple providers. Such `multi-homed' routing
domains may, for example, consist of single-site campuses and
companies which are attached to multiple backbones, large
organizations which are attached to different providers at different
locations in the same country, or multi-national organizations which
are attached to backbones in a variety of countries worldwide. There
are a number of possible ways to deal with these multi-homed routing
domains.
One possible solution is to assign addresses to each multi-homed
organization independently from the providers to which it is
attached. This allows each multi-homed organization to base its IP
assignments on a single prefix, and to thereby summarize the set of
all IP addresses reachable within that organization via a single
prefix. The disadvantage of this approach is that since the IP
address for that organization has no relationship to the addresses of
any particular TRD, the TRDs to which this organization is attached
will need to advertise the prefix for this organization to other
providers. Other providers (potentially worldwide) will need to
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maintain an explicit entry for that organization in their routing
tables.
For example, suppose that a very large North American company `Mega
Big International Incorporated' (MBII) has a fully interconnected
internal network and is assigned a single prefix as part of the North
American prefix. It is likely that outside of North America, a
single entry may be maintained in routing tables for all North
American Destinations. However, within North America, every provider
will need to maintain a separate address entry for MBII. If MBII is
in fact an international corporation, then it may be necessary for
every provider worldwide to maintain a separate entry for MBII
(including backbones to which MBII is not attached). Clearly this may
be acceptable if there are a small number of such multi-homed routing
domains, but would place an unacceptable load on routers within
backbones if all organizations were to choose such address
assignments. This solution may not scale to internets where there
are many hundreds of thousands of multi-homed organizations.
A second possible approach would be for multi-homed organizations to
be assigned a separate IP address space for each connection to a TRD,
and to assign a single prefix to some subset of its domain(s) based
on the closest interconnection point. For example, if MBII had
connections to two providers in the U.S. (one east coast, and one
west coast), as well as three connections to national backbones in
Europe, and one in the far east, then MBII may make use of six
different address prefixes. Each part of MBII would be assigned a
single address prefix based on the nearest connection.
For purposes of external routing of traffic from outside MBII to a
destination inside of MBII, this approach works similarly to treating
MBII as six separate organizations. For purposes of internal routing,
or for routing traffic from inside of MBII to a destination outside
of MBII, this approach works the same as the first solution.
If we assume that incoming traffic (coming from outside of MBII, with
a destination within MBII) is always to enter via the nearest point
to the destination, then each TRD which has a connection to MBII
needs to announce to other TRDs the ability to reach only those parts
of MBII whose address is taken from its own address space. This
implies that no additional routing information needs to be exchanged
between TRDs, resulting in a smaller load on the inter-domain routing
tables maintained by TRDs when compared to the first solution. This
solution therefore scales better to extremely large internets
containing very large numbers of multi-homed organizations.
One problem with the second solution is that backup routes to multi-
homed organizations are not automatically maintained. With the first
solution, each TRD, in announcing the ability to reach MBII,
specifies that it is able to reach all of the hosts within MBII. With
the second solution, each TRD announces that it can reach all of the
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hosts based on its own address prefix, which only includes some of
the hosts within MBII. If the connection between MBII and one
particular TRD were severed, then the hosts within MBII with
addresses based on that TRD would become unreachable via inter-domain
routing. The impact of this problem can be reduced somewhat by
maintenance of additional information within routing tables, but this
reduces the scaling advantage of the second approach.
The second solution also requires that when external connectivity
changes, internal addresses also change.
Also note that this and the previous approach will tend to cause
packets to take different routes. With the first approach, packets
from outside of MBII destined for within MBII will tend to enter via
the point which is closest to the source (which will therefore tend
to maximize the load on the networks internal to MBII). With the
second solution, packets from outside destined for within MBII will
tend to enter via the point which is closest to the destination
(which will tend to minimize the load on the networks within MBII,
and maximize the load on the TRDs).
These solutions also have different effects on policies. For example,
suppose that country `X' has a law that traffic from a source within
country X to a destination within country X must at all times stay
entirely within the country. With the first solution, it is not
possible to determine from the destination address whether or not the
destination is within the country. With the second solution, a
separate address may be assigned to those hosts which are within
country X, thereby allowing routing policies to be followed.
Similarly, suppose that `Little Small Company' (LSC) has a policy
that its packets may never be sent to a destination that is within
MBII. With either solution, the routers within LSC may be configured
to discard any traffic that has a destination within MBII's address
space. However, with the first solution this requires one entry; with
the second it requires many entries and may be impossible as a
practical matter.
There are other possible solutions as well. A third approach is to
assign each multi-homed organization a single address prefix, based
on one of its connections to a TRD. Other TRDs to which the multi-
homed organization are attached maintain a routing table entry for
the organization, but are extremely selective in terms of which other
TRDs are told of this route. This approach will produce a single
`default' routing entry which all TRDs will know how to reach (since
presumably all TRDs will maintain routes to each other), while
providing more direct routing in some cases.
There is at least one situation in which this third approach is
particularly appropriate. Suppose that a special interest group of
organizations have deployed their own backbone. For example, lets
suppose that the U.S. National Widget Manufacturers and Researchers
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have set up a U.S.-wide backbone, which is used by corporations who
manufacture widgets, and certain universities which are known for
their widget research efforts. We can expect that the various
organizations which are in the widget group will run their internal
networks as separate routing domains, and most of them will also be
attached to other TRDs (since most of the organizations involved in
widget manufacture and research will also be involved in other
activities). We can therefore expect that many or most of the
organizations in the widget group are dual-homed, with one attachment
for widget-associated communications and the other attachment for
other types of communications. Let's also assume that the total
number of organizations involved in the widget group is small enough
that it is reasonable to maintain a routing table containing one
entry per organization, but that they are distributed throughout a
larger internet with many millions of (mostly not widget-associated)
routing domains.
With the third approach, each multi-homed organization in the widget
group would make use of an address assignment based on its other
attachment(s) to TRDs (the attachments not associated with the widget
group). The widget backbone would need to maintain routes to the
routing domains associated with the various member organizations.
Similarly, all members of the widget group would need to maintain a
table of routes to the other members via the widget backbone.
However, since the widget backbone does not inform other general
worldwide TRDs of what addresses it can reach (since the backbone is
not intended for use by other outside organizations), the relatively
large set of routing prefixes needs to be maintained only in a
limited number of places. The addresses assigned to the various
organizations which are members of the widget group would provide a
`default route' via each members other attachments to TRDs, while
allowing communications within the widget group to use the preferred
path.
A fourth solution involves assignment of a particular address prefix
for routing domains which are attached to precisely two (or more)
specific routing domains. For example, suppose that there are two
providers `SouthNorthNet' and `NorthSouthNet' which have a very large
number of customers in common (i.e., there are a large number of
routing domains which are attached to both). Rather than getting two
address prefixes these organizations could obtain three prefixes.
Those routing domains which are attached to NorthSouthNet but not
attached to SouthNorthNet obtain an address assignment based on one
of the prefixes. Those routing domains which are attached to
SouthNorthNet but not to NorthSouthNet would obtain an address based
on the second prefix. Finally, those routing domains which are
multi-homed to both of these networks would obtain an address based
on the third prefix. Each of these two TRDs would then advertise two
prefixes to other TRDs, one prefix for leaf routing domains attached
to it only, and one prefix for leaf routing domains attached to both.
Expiration Date March 1993 [Page 15]
This fourth solution is likely to be important when use of public
data networks becomes more common. In particular, it is likely that
at some point in the future a substantial percentage of all routing
domains will be attached to public data networks. In this case,
nearly all government-sponsored networks (such as some current NSFNET
regionals) may have a set of customers which overlaps substantially
with the public networks.
There are therefore a number of possible solutions to the problem of
assigning IP addresses to multi-homed routing domains. Each of these
solutions has very different advantages and disadvantages. Each
solution places a different real (i.e., financial) cost on the
multi-homed organizations, and on the TRDs (including those to which
the multi-homed organizations are not attached).
In addition, most of the solutions described also highlight the need
for each TRD to develop policy on whether and under what conditions
to accept addresses that are not based on its own address prefix, and
how such non-local addresses will be treated. For example, a somewhat
conservative policy might be that non-local IP address prefixes will
be accepted from any attached leaf RD, but not advertised to other
TRDs. In a less conservative policy, a TRD might accept such non-
local prefixes and agree to exchange them with a defined set of other
TRDs (this set could be an a priori group of TRDs that have something
in common such as geographical location, or the result of an
agreement specific to the requesting leaf RD). Various policies
involve real costs to TRDs, which may be reflected in those policies.
5.5 Private Links
The discussion up to this point concentrates on the relationship
between IP addresses and routing between various routing domains over
transit routing domains, where each transit routing domain
interconnects a large number of routing domains and offers a more-
or-less public service.
However, there may also exist a large number of private point-to-
point links which interconnect two private routing domains. In many
cases such private point-to-point links may be limited to forwarding
packets directly between the two private routing domains.
For example, let's suppose that the XYZ corporation does a lot of
business with MBII. In this case, XYZ and MBII may contract with a
carrier to provide a private link between the two corporations, where
this link may only be used for packets whose source is within one of
the two corporations, and whose destination is within the other of
the two corporations. Finally, suppose that the point-to-point link
is connected between a single router (router X) within XYZ
corporation and a single router (router M) within MBII. It is
Expiration Date March 1993 [Page 16]
therefore necessary to configure router X to know which addresses can
be reached over this link (specifically, all addresses reachable in
MBII). Similarly, it is necessary to configure router M to know which
addresses can be reached over this link (specifically, all addresses
reachable in XYZ Corporation).
The important observation to be made here is that such private links
may be ignored for the purpose of IP address allocation, and do not
pose a problem for routing. This is because the routing information
associated with private links is not propagated throughout the
internet, and therefore does not need to be collapsed into a TRD's
prefix.
In our example, let's suppose that the XYZ corporation has a single
connection to an NSFNET regional, and has therefore received an
address allocation from the space administered by that regional.
Similarly, let's suppose that MBII, as an international corporation
with connections to six different providers, has chosen the second
solution from Section 5.4, and therefore has obtained six different
address allocations. In this case, all addresses reachable in the XYZ
Corporation can be described by a single address prefix (implying
that router M only needs to be configured with a single address
prefix to represent the addresses reachable over this point-to-point
link). All addresses reachable in MBII can be described by six
address prefixes (implying that router X needs to be configured with
six address prefixes to represent the addresses reachable over the
point-to-point link).
In some cases, such private point-to-point links may be permitted to
forward traffic for a small number of other routing domains, such as
closely affiliated organizations. This will increase the
configuration requirements slightly. However, provided that the
number of organizations using the link is relatively small, then this
still does not represent a significant problem.
Note that the relationship between routing and IP addressing
described in other sections of this paper is concerned with problems
in scaling caused by large, essentially public transit routing
domains which interconnect a large number of routing domains.
However, for the purpose of IP address allocation, private point-to-
point links which interconnect only a small number of private routing
domains do not pose a problem, and may be ignored. For example, this
implies that a single leaf routing domain which has a single
connection to a `public' backbone (e.g., the NSFNET), plus a number
of private point-to-point links to other leaf routing domains, can be
treated as if it were single-homed to the backbone for the purpose of
IP address allocation. We expect that this is also another way of
dealing with multi-homed domains.
Expiration Date March 1993 [Page 17]
5.6 Zero-Homed Routing Domains
Currently, a very large number of organizations have internal
communications networks which are not connected to any external
network. Such organizations may, however, have a number of private
point-to-point links that they use for communications with other
organizations. Such organizations do not participate in global
routing, but are satisfied with reachability to those organizations
with which they have established private links. These are referred to
as zero-homed routing domains.
Zero-homed routing domains can be considered as the degenerate case
of routing domains with private links, as discussed in the previous
section, and do not pose a problem for inter-domain routing. As
above, the routing information exchanged across the private links
sees very limited distribution, usually only to the RD at the other
end of the link. Thus, there are no address abstraction requirements
beyond those inherent in the address prefixes exchanged across the
private link.
However, it is important that zero-homed routing domains use valid
globally unique IP addresses. Suppose that the zero-homed routing
domain is connected through a private link to an RD. Further, this RD
participates in an internet that subscribes to the global IP
addressing plan. This RD must be able to distinguish between the
zero-homed routing domain's IP addresses and any other IP addresses
that it may need to route to. The only way this can be guaranteed is
if the zero-homed routing domain uses globally unique IP addresses.
5.7 Transition Issues
Allocation of IP addresses based on connectivity to TRDs is important
to allow scaling of inter-domain routing to an internet containing
millions of routing domains. However, such address allocation based
on topology also implies that a change in topology may result in a
change of address.
Note that an address change need not happen immediately. A domain
which has changed service providers may still advertise its prefix
through its new service provider. Since upper levels in the routing
hierarchy will perform routing based on the longest prefix,
reachability is preserved, although the aggregation and scalability
of the routing information has greatly diminished. Thus, a domain
which does change its topology should change addresses as soon as
convenient. The timing and mechanics of such changes must be the
result of a multilateral agreement between the old service provider,
the new provider, and the domain.
Expiration Date March 1993 [Page 18]
This need to allow for change in addresses is a natural, inevitable
consequence of routing data abstraction. The basic notion of routing
data abstraction is that there is some correspondence between the
address and where a system (i.e., a routing domain, subnetwork, or
end system) is located. Thus if the system moves, in some cases the
address will have to change. If it were possible to change the
connectivity between routing domains without changing the addresses,
then it would clearly be necessary to keep track of the location of
that routing domain on an individual basis.
In the short term, due to the rapid growth and increased
commercialization of the Internet, it is possible that the topology
may be relatively volatile. This implies that planning for address
transition is very important. Fortunately, there are a number of
steps which can be taken to help ease the effort required for address
transition. A complete description of address transition issues is
outside of the scope of this paper. However, a very brief outline of
some transition issues is contained in this section.
Also note that the possible requirement to transition addresses based
on changes in topology imply that it is valuable to anticipate the
future topology changes before finalizing a plan for address
allocation. For example, in the case of a routing domain which is
initially single-homed, but which is expecting to become multi-homed
in the future, it may be advantageous to assign IP addresses based on
the anticipated future topology.
In general, it will not be practical to transition the IP addresses
assigned to a routing domain in an instantaneous `change the address
at midnight' manner. Instead, a gradual transition is required in
which both the old and the new addresses will remain valid for a
limited period of time. During the transition period, both the old
and new addresses are accepted by the end systems in the routing
domain, and both old and new addresses must result in correct routing
of packets to the destination.
During the transition period, it is important that packets using the
old address be forwarded correctly, even when the topology has
changed. This is facilitated by the use of `longest match' inter-
domain routing.
For example, suppose that the XYZ Corporation was previously
connected only to the NorthSouthNet NSFNET regional. The XYZ
Corporation therefore went off to the NorthSouthNet administration
and got an IP address prefix assignment based on the IP address
prefix value assigned to the NorthSouthNet regional. However, for a
variety of reasons, the XYZ Corporation decided to terminate its
association with the NorthSouthNet, and instead connect directly to
the NewCommercialNet public data network. Thus the XYZ Corporation
now has a new address assignment under the IP address prefix assigned
to the NewCommercialNet. The old address for the XYZ Corporation
Expiration Date March 1993 [Page 19]
would seem to imply that traffic for the XYZ Corporation should be
routed to the NorthSouthNet, which no longer has any direct
connection with XYZ Corporation.
If the old TRD (NorthSouthNet) and the new TRD (NewCommercialNet) are
adjacent and cooperative, then this transition is easy to accomplish.
In this case, packets routed to the XYZ Corporation using the old
address assignment could be routed to the NorthSouthNet, which would
directly forward them to the NewCommercialNet, which would in turn
forward them to XYZ Corporation. In this case only NorthSouthNet and
NewCommercialNet need be aware of the fact that the old address
refers to a destination which is no longer directly attached to
NorthSouthNet.
If the old TRD and the new TRD are not adjacent, then the situation
is a bit more complex, but there are still several possible ways to
forward traffic correctly.
If the old TRD and the new TRD are themselves connected by other
cooperative transit routing domains, then these intermediate domains
may agree to forward traffic for XYZ correctly. For example, suppose
that NorthSouthNet and NewCommercialNet are not directly connected,
but that they are both directly connected to the NSFNET backbone. In
this case, all three of NorthSouthNet, NewCommercialNet, and the
NSFNET backbone would need to maintain a special entry for XYZ
corporation so that traffic to XYZ using the old address allocation
would be forwarded via NewCommercialNet. However, other routing
domains would not need to be aware of the new location for XYZ
Corporation.
Suppose that the old TRD and the new TRD are separated by a non-
cooperative routing domain, or by a long path of routing domains. In
this case, the old TRD could encapsulate traffic to XYZ Corporation
in order to deliver such packets to the correct backbone.
Also, those locations which do a significant amount of business with
XYZ Corporation could have a specific entry in their routing tables
added to ensure optimal routing of packets to XYZ. For example,
suppose that another commercial backbone ``OldCommercialNet'' has a
large number of customers which exchange traffic with XYZ
Corporation, and that this third TRD is directly connected to both
NorthSouthNet and NewCommercialNet. In this case OldCommercialNet
will continue to have a single entry in its routing tables for other
traffic destined for NorthSouthNet, but may choose to add one
additional (more specific) entry to ensure that packets sent to XYZ
Corporation's old address are routed correctly.
Whichever method is used to ease address transition, the goal is that
knowledge relating XYZ to its old address that is held throughout the
global internet would eventually be replaced with the new
information. It is reasonable to expect this to take weeks or months
Expiration Date March 1993 [Page 20]
and will be accomplished through the distributed directory system.
Discussion of the directory, along with other address transition
techniques such as automatically informing the source of a changed
address, are outside the scope of this paper.
Another significant transition difficulty is the establishment of
appropriate addressing authorities. In order not to delay the
deployment of this addressing scheme, if no authority has been
created at an appropriate level, a higher level authority may
allocated addresses instead of the lower level authority. For
example, suppose that the continental authority has been allocated a
portion of the address space and that the service providers present
on that continent are clear, but have not yet established their
addressing authority. The continental authority may foresee
(possibly with information from the provider) that the provider will
eventually create an authority. The continental authority may then
act on behalf of that provider until the provider is prepared to
assume its addressing authority duties.
5.8 Interaction with Policy Routing
We assume that any inter-domain routing protocol will have difficulty
trying to aggregate multiple destinations with dissimilar policies.
At the same time, the ability to aggregate routing information while
not violating routing policies is essential. Therefore, we suggest
that address allocation authorities attempt to allocate addresses so
that aggregates of destinations with similar policies can be easily
formed.
6 Recommendations
We anticipate that the current exponential growth of the Internet
will continue or accelerate for the foreseeable future. In addition,
we anticipate a rapid internationalization of the Internet. The
ability of routing to scale is dependent upon the use of data
abstraction based on hierarchical IP addresses. As CIDR [1] is
introduced in the Internet, it is therefore essential to choose a
hierarchical structure for IP addresses with great care.
It is in the best interests of the internetworking community that the
cost of operations be kept to a minimum where possible. In the case
of IP address allocation, this again means that routing data
abstraction must be encouraged.
In order for data abstraction to be possible, the assignment of IP
addresses must be accomplished in a manner which is consistent with
the actual physical topology of the Internet. For example, in those
Expiration Date March 1993 [Page 21]
cases where organizational and administrative boundaries are not
related to actual network topology, address assignment based on such
organization boundaries is not recommended.
The intra-domain routing protocols allow for information abstraction
to be maintained within a domain. For zero-homed and single-homed
routing domains (which are expected to remain zero-homed or single-
homed), we recommend that the IP addresses assigned within a single
routing domain use a single address prefix assigned to that domain.
Specifically, this allows the set of all IP addresses reachable
within a single domain to be fully described via a single prefix.
We anticipate that the total number of routing domains existing on a
worldwide Internet to be great enough that additional levels of
hierarchical data abstraction beyond the routing domain level will be
necessary.
In most cases, network topology will have a close relationship with
national boundaries. For example, the degree of network connectivity
will often be greater within a single country than between countries.
It is therefore appropriate to make specific recommendations based on
national boundaries, with the understanding that there may be
specific situations where these general recommendations need to be
modified.
6.1 Recommendations for an address allocation plan
We anticipate that public interconnectivity between private routing
domains will be provided by a diverse set of TRDs, including (but not
necessarily limited to):
- backbone networks (NSFNET, ANSnet, CIX, PSI, SprintLink,
Ebone);
- a number of regional or national networks; and,
- a number of commercial Public Data Networks.
It is also expected that these networks will not be interconnected in
a strictly hierarchical manner (for example, there is expected to be
direct connectivity between NSFNET regionals, and all four of these
types of networks may have direct international connections).
However, the total number of such TRDs is expected to remain (for the
foreseeable future) small enough to allow addressing of this set of
TRDs via a flat address space. These TRDs will be used to
interconnect a wide variety of routing domains, each of which may
comprise a single corporation, part of a corporation, a university
Expiration Date March 1993 [Page 22]
campus, a government agency, or other organizational unit.
In addition, some private corporations may be expected to make use of
dedicated private TRDs for communication within their own
corporation.
We anticipate that the great majority of routing domains will be
attached to only one of the TRDs. This will permit hierarchical
address aggregation based on TRD. We therefore strongly recommend
that addresses be assigned hierarchically, based on address prefixes
assigned to individual TRDs.
To support continental aggregation of routes, we recommend that all
addresses for TRDs which are wholly within a continent be taken from
the continental prefix.
For the proposed address allocation scheme, this implies that
administrative authority for some prefix should be assigned to all
TRDs (explicitly including the backbones and NSFNET regionals). For
those leaf routing domains which are connected to a single TRD, they
should be assigned a prefix value from the address space assigned to
that TRD.
We recommend that all TRDs explicitly be involved in the task of
address administration for those leaf routing domains which are
single-homed to them. This will offer a valuable service to their
customers, and will also greatly reduce the resources (including
human and network resources) necessary for that TRD to take part in
inter-domain routing.
Each TRD should develop policy on whether and under what conditions
to accept addresses that are not based on its own address prefix, and
how such non-local addresses will be treated. Policies should reflect
the issue of cost associated with implementing such policies.
For routing domains which are not attached to any publically
available TRD, there is not the same urgent need for hierarchical
address abbreviation. We do not, therefore, make any additional
recommendations for such `isolated' routing domains. Where such
domains are connected to other domains by private point-to-point
links, and where such links are used solely for routing between the
two domains that they interconnect, again no additional technical
problems relating to address abbreviation is caused by such a link,
and no specific additional recommendations are necessary.
Further, in order to allow aggregation of IP addresses at national
and continental boundaries into as few prefixes as possible, we
further recommend that IP addresses allocated to routing domains
should be assigned based on each routing domain's connectivity to
national and continental Internet backbones.
Expiration Date March 1993 [Page 23]
6.2 Recommendations for Multi-Homed Routing Domains
Some routing domains will be attached to multiple TRDs within the
same country, or to TRDs within multiple different countries. We
refer to these as `multi-homed' routing domains. Clearly the strict
hierarchical model discussed above does not neatly handle such
routing domains.
There are several possible ways that these multi-homed routing
domains may be handled. Each of these methods vary with respect to
the amount of information that must be maintained for inter-domain
routing and also with respect to the inter-domain routes. In
addition, the organization that will bear the brunt of this cost
varies with the possible solutions. For example, the solutions vary
with respect to:
- resources used within routers within the TRDs;
- administrative cost on TRD personnel; and,
- difficulty of configuration of policy-based inter-domain
routing information within leaf routing domains.
Also, the solution used may affect the actual routes which packets
follow, and may effect the availability of backup routes when the
primary route fails.
For these reasons it is not possible to mandate a single solution for
all situations. Rather, economic considerations will require a
variety of solutions for different routing domains, service
providers, and backbones.
6.3 Recommendations for the Administration of IP addresses.
With the growth of the Internet and its increasing globalization,
much thought has been given to the evolution of the network number
allocation and assignment process. RFC 1174, "Identifier Assignment
and Connected Status",[3] dated August 1990 recommends that the Internet
Registry (IR) continue as the principal registry for network numbers;
however, the IR may allocate blocks of network numbers and the
assignment of those numbers to qualified organizations. The IR will
serve as the default registry in cases where no delegated
registration authority has been identified.
The distribution of the registration function is desirable, and in
keeping with that goal, it is necessary to develop a plan which
Expiration Date March 1993 [Page 24]
manages the distribution of the network number space. The demand for
network numbers has grown significantly within the last two years and
as a result the allocation of network numbers must be approached in a
more systematic fashion.
6.3.1 Qualifications for Distributed Regional Registries
The major reason to distribute the registration function is that the
Internet serves a more diverse global population than it did at its
inception. This means that registries which are located in distinct
geographic areas may be better able to serve the local community in
terms of language and local customs. While there appears to be wide
support for the concept of distribution of the registration function,
it is important to define how the candidate delegated registries will
be chosen and from which geographic areas.
Based on the growth and the maturity of the Internet in Europe,
Central/South America and the Pacific Rim areas, it is desirable to
consider delegating the registration function to an organization in
each of those geographic areas. Until an organization is identified
in those regions, the IR will continue to serve as the default
registry. The IR remains the root registry and continues to provide
the registration function to all those regions not covered by
distributed regional registries. And as other regions of the world
become more and more active in the Internet, the IANA and the IR may
choose to look for candidate registries to serve the populations in
those geographic regions.
It is important that the regional registry is unbiased and and widely
recognized by network providers and subscribers within the geographic
region. It is also important that there is just a single regional
registry per geographical region at this level to provide for
efficient and fair sub-allocation of the address space to service
providers and subscribers. To be selected as a distributed regional
registry an organization should meet the following criteria:
- networking authorities within the geographic area legitimize
the organization
- the organization is well-established and has legitimacy outside
of the registry function
- the organization will commit appropriate resources to provide
stable, timely, and reliable service to the geographic region
Expiration Date March 1993 [Page 25]
- the commitment to allocate IP numbers according to the
guidelines established by the IANA and the IR
- the commitment to coordinate with the IR to establish
qualifications and strategies for sub-allocations of the
regional allocation.
The distributed regional registry is empowered by the IANA and the IR
to provide the network number registration function to a geographic
area. It is possible for network subscribers to contact the IR
directly. Depending on the circumstances the network subscriber may
be referred to the regional registry, but the IR will be prepared to
service any network subscriber if necessary.
6.3.2 Allocation of the Network Number Space by the Internet Registry
The Class A portion of the number space represents 50% of the total
IP numbers; Class B is 25% of the total; Class C is approximately 12%
of the total. Table 1 shows the current allocation of the IP network
numbers.
Total Allocated Allocated (%)
Class A 126 49 38%
Class B 16383 7354 45%
Class C 2097151 44014 2%
Table 1: Network Number Statistics (June 1992) [4]
Class A and B network numbers are a limited resource and therefore
the entire number space will be retained by the IR. No allocations
from the Class A and B network numbers will be made to distributed
regional registries at this time.
The Class C network number space will be divided into allocatable
blocks which will be reserved by the IANA and IR for allocation to
distributed regional registries. In the absence of designated
regional registries in geographic areas, the IR will assign addresses
to networks within those geographic areas according to the Class C
allocation divisions.
A preliminary inspection of the Class C IP network numbers shows that
the number space with prefix 192 is assigned. The remaining space
Expiration Date March 1993 [Page 26]
from prefix 193 through 223 is mostly unassigned.
The IANA and the IR will reserve the upper half of this reserved
space which corresponds to the IP address range of 208.0.0.0 through
223.255.255.255. Network numbers from this portion of the Class C space
will remain unallocated and unassigned until further notice.
The remaining Class C network number space will be allocated in a
fashion which is compatible with potential address aggregation
techniques. It is intended to divide this address range into eight
equally sized address blocks.
192.0.0.0 - 193.255.255.255
194.0.0.0 - 195.255.255.255
196.0.0.0 - 197.255.255.255
198.0.0.0 - 199.255.255.255
200.0.0.0 - 201.255.255.255
202.0.0.0 - 203.255.255.255
204.0.0.0 - 205.255.255.255
206.0.0.0 - 207.255.255.255
Each block represents 131,072 addresses or approximately 6% of the
total Class C address space.
It is proposed that a broad geographic allocation be used for these
blocks. At present there are four major areas of address allocation:
Europe, North America, Pacific Rim, and South & Central America.
We propose that the top level block allocation be designated as
follows:
Multi-regional 192.0.0.0 - 193.255.255.255
Europe 194.0.0.0 - 195.255.255.255
Others 196.0.0.0 - 197.255.255.255
North America 198.0.0.0 - 199.255.255.255
Central/South
America 200.0.0.0 - 201.255.255.255
Pacific Rim 202.0.0.0 - 203.255.255.255
Others 204.0.0.0 - 205.255.255.255
Others 206.0.0.0 - 207.255.255.255
It is proposed that the IR, and any designated regional registries,
allocate addresses in conformance with this overall scheme. Where
Expiration Date March 1993 [Page 27]
there are qualifying regional registries established, primary
responsibility for allocation from within that block will be
delegated to that registry.
The ranges designated as "Others" permit flexibility in network
number assignments which are outside of the geographical regions
already allocated. The range listed as multi-regional represents
network numbers which have been assigned prior to the implementation
of this plan. We propose that the IANA and the IR will adopt these
divisions of the Class C network number space and will begin
assigning network numbers accordingly.
6.3.3 Assignment of the Network Number Space
The exhaustion of the IP address space is a topic of concern for the
entire Internet community. This plan for the assignment of Class A,
B, or C IP numbers to network subscribers has two major goals:
1) to reserve a portion of the IP number space so that it may be
available for transition to a new numbering plan
2) to assign the Class C network number space in a fashion which is
compatible with proposed address aggregation techniques
6.3.3.1 Class A
The Class A number space can support the largest number of unique
host identifier addresses and is also the class of network numbers
most sparsely populated. There are only approximately 77 Class A
network numbers which are unassigned, and these 77 network numbers
represent about 30% of the total network number space.
The IANA will retain sole responsibility for the assignment of Class
A network numbers. The upper half of the Class A number space will be
reserved indefinitely (IP network addresses 64.0.0.0 through
127.0.0.0). While it is expected that no new assignments of Class A
numbers will take place in the near future, any organization
petitioning the IANA for a Class A network number will be expected to
provide a detailed technical justification documenting network size
and structure. Class A assignments are at the IANA's discretion.
6.3.3.2 Class B
Previously organizations were recommended to use a subnetted Class B
network number rather than multiple Class C network numbers. Due to
Expiration Date March 1993 [Page 28]
the scarcity of Class B network numbers and the under utilization of
the Class B number space by most organizations, the recommendation is
now to use multiple Class Cs where practical.
The IANA and the IR will maintain sole responsibility for the Class B
number space. Where there are designated regional registries, those
registries will be a valuable source of information to the IR in
evaluating requests for Class B numbers. Organizations applying for
a Class B network number should fulfill the following criteria:
- the organization presents a subnetting plan which documents
more than 32 subnets within its organizational network, AND
- the organization has more than 4096 hosts.
These criteria assume that an organization which meets this profile
will continue to grow and that assigning a Class B network number to
them will permit network growth and reasonable utilization of the
assigned number space.
6.3.3.3 Class C
This document recommends a division of the Class C number space.
That division is primarily an administrative division which lays the
groundwork for distributed network number registries. This section
deals with how network numbers are assigned from within those blocks.
Sub-allocations of the block to sub-registries is beyond the scope of
this paper.
By default, if an organization requires more than a single Class C,
it will be assigned a bit-wise contiguous block from the Class C
space allocated for its geographic region.
For instance, an European organization which requires fewer than 2048
unique IP addresses and more than 1024 would be assigned 8 contiguous
class C network numbers from the number space reserved for European
networks, 194.0.0.0 - 195.255.255.255. If an organization from
Central America required fewer than 512 unique IP addresses and more
than 256, it would receive 2 contiguous class C network numbers from
the number space reserved for Central/South American networks,
200.0.0.0 - 201.255.255.255.
The IR or the registry to whom the IR has delegated the registration
function will determine the number of Class C network numbers to
assign to a network subscriber based on the following criteria:
Expiration Date March 1993 [Page 29]
Organization Assignment
1) requires fewer than 256 addresses 1 class C network
2) requires fewer than 512 addresses 2 contiguous class C networks
3) requires fewer than 1024 addresses 4 contiguous class C networks
4) requires fewer than 2048 addresses 8 contiguous class C networks
5) requires fewer than 4096 addresses 16 contiguous class C networks
The number of addresses that a network subscriber indicates that it
needs should be based on a 24 month projection.
The maximal block of class C nets that should be assigned to a
subscriber consists of sixteen contiguous class C networks which
corresponds to a single IP prefix the length of which is twelve bits.
If a subscriber has a requirement for more than 4096 unique network
identifiers it should most likely receive a Class B net number.
7 Security Considerations
Security issues are not discussed in this memo.
8 Acknowledgments
The authors would like to acknowledge the substantial contributions
made by the authors of RFC 1237 [2], Richard Colella, Ella Gardner,
and Ross Callon. The significant concepts (and much of the text) in
this memo are taken directly from their work.
The authors would like to acknowledge the substantial contributions
made by the members of the following two groups, the Federal
Engineering Planning Group (FEPG) and the International Engineering
Planning Group (IEPG). This document also reflects many concepts
expressed at the IETF Addressing BOF which took place in Cambridge,
MA in July 1992.
We would also like to thank Kannan Varadhan (OARnet), Jon Postel
(ISI), Bernhard Stockman (NORDUNET/SUNET), Claudio Topologic (CNRI),
Barry Leiner (ADS), and Peter Ford (Los Alamos National Laboratory)
for their review and constructive comments.
Expiration Date March 1993 [Page 30]
9 Authors' Addresses
Yakov Rekhter
T.J. Watson Research Center IBM Corporation
P.O. Box 218
Yorktown Heights, NY 10598
Phone: (914) 945-3896
email: yakov@watson.ibm.com
Tony Li
cisco Systems, Inc.
1525 O'Brien Drive
Menlo Park, CA 94025
email: tli@cisco.com
Elise Gerich
Merit Network Inc.
1071 Beal Avenue
Ann Arbor, MI 48109
email: epg@merit.edu
10 References
[1] Fuller, V., Li, T., Yu, J., and Varadhan, K., `Supernetting: an
Address Assignment and Aggregation Strategy', RFC1338, June 1992
[2] Colella, R., Gardner, E, and Callon, R., `Guidelines for OSI NSAP
Allocation in the Internet'. RFC1237, July 1991
[3] Cerf, V., 'IAB Recommended Policy on Distributing
Internet Identifier Assignment and IAB Recommended Policy
Change to Internet "Connected" Status', RFC1174, August 1990
[4] Wang, Z., Crowcroft, J., Working Draft 'A Two-Tier Address
Structure for the Internet: a solution to the problem of
address space exhaustion', May 1992
Expiration Date March 1993 [Page 31]