Network Working Group Y. Rekhter
Cisco Systems
B. Moskowitz
Chrysler Corp.
D. Karrenberg
RIPE NCC
G. J. de Groot
RIPE NCC
E. Lear
Silicon Graphics, Inc.
July 1995
Address Allocation for Private Internets
<draft-ietf-cidrd-private-addr-01.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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ftp.isi.edu (US West Coast).
1. Introduction
For the purposes of this document, an enterprise is an entity
autonomously operating a network using TCP/IP and in particular
determining the addressing plan and address assignments within that
network.
This document describes address allocation for private internets. The
allocation permits full network layer connectivity between all hosts
inside an enterprise as well as between all public hosts of different
enterprises. The cost of using private internet address space is the
potentially costly effort to renumber hosts and networks between
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public and private.
2. Motivation
With the proliferation of TCP/IP technology worldwide, including
outside the Internet itself, an increasing number of non-connected
enterprises use this technology and its addressing capabilities for
sole intra-enterprise communications, without any intention to ever
directly connect to other enterprises or the Internet itself.
The Internet has grown beyond anyone's expectations. Sustained
exponential growth continues to introduce new challenges [CITE growth
ietf proceedings)]. One challenge is a concern within the community
that globally unique address space will be exhausted. A separate and
far more pressing concern is that the amount of routing overhead will
grow beyond the capabilities of Internet Service Providers. Efforts
progress within the community to find long term solutions to both
these problems. Meanwhile it is necessary to revisit address
allocation procedures, and their impact on the Internet routing
system.
Acquiring globally unique addresses from an Internet registry is no
longer sufficient to achieve Internet-wide IP connectivity. In the
past assignment of globally unique addresses had been sufficient to
insure Internet-wide reachability to these addresses. To contain
growth of routing overhead, an Internet Provider obtains a block of
address space from an address registry, and then assigns to its
customers addresses from within that block based on each customer
requirement. The result of this process is that routes to many
customers will appear to other providers as a single route [RFC1518],
[RFC1519].
In order for route aggregation to be effective, Internet providers
encourage customers joining their network to use the provider's
block, and thus renumber their computers. Such encouragement may
become a requirement in the future. With the current size of the
Internet and its growth rate it is no longer realistic to assume that
by virtue of acquiring globally unique IP addresses out of an
Internet registry an organization that acquires such addresses would
have Internet-wide IP connectivity once the organization gets
connected to the Internet. To the contrary, it is quite likely that
when the organization would connect to the Internet to achieve
Internet-wide IP connectivity the organization would need to change
IP addresses (renumber) all of its public hosts (hosts that require
Internet-wide IP connectivity), regardless of whether the addresses
used by the organization initially were globally unique or not.
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The current practice is to assign globally unique addresses to all
hosts that use TCP/IP. In order to extend the life of the IPv4
address space, address registries are requiring more justification
than ever before, making it harder for organizations to acquire
additional address space [RFC1466].
Hosts within enterprises that use IP can be partitioned into three
categories:
Category 1: hosts that do not require access to hosts in other
enterprises or the Internet at large; hosts within
this category may use IP addresses that are unambiguous
within an enterprise, but may be ambiguous between
enterprises.
Category 2: hosts that need access to a limited set of outside
services (e.g., E-mail, FTP, netnews, remote login)
which can be handled by application layer gateways.
for many hosts in this category an unrestricted external
access (provided via IP connectivity) may be unnecessary
and even undesirable for privacy/security reasons.
Just like hosts within the first category, such hosts
may use IP addresses that are unambiguous within an
enterprise, but may be ambiguous between enterprises.
Category 3: hosts that need network layer access outside the
enterprise (provided via IP connectivity); hosts in
the last category require IP addresses that are globally
unambiguous.
We will refer to the hosts in the first and second categories as
"private". We will refer to the hosts in the third category as
"public".
Many applications require connectivity only within one enterprise and
do not need external (outside the enterprise) connectivity for the
majority of internal hosts. In larger enterprises it is often easy
to identify a substantial number of hosts using TCP/IP that do not
need network layer connectivity outside the enterprise.
Some examples, where external connectivity might not be required,
are:
- A large airport which has its arrival/departure displays
individually addressable via TCP/IP. It is very unlikely that
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these displays need to be directly accessible from other
networks.
- Large organizations like banks and retail chains are switching
to TCP/IP for their internal communication. Large numbers of
local workstations like cash registers, money machines, and
equipment at clerical positions rarely need to have such
connectivity.
- For security reasons, many enterprises use application layer
gateways (e.g., firewalls) to connect their internal network to
the Internet. The internal network usually does not have direct
access to the Internet, thus only one or more firewall hosts are
visible from the Internet. In this case, the internal network
can use non-unique IP numbers.
- Interfaces of routers on an internal network usually do not
need to be directly accessible from outside the enterprise.
3. Private Address Space
The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for private internets:
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
We will refer to the first block as "24-bit block", the second as
"20-bit block, and to the third as "16-bit" block. Note that the
first block is nothing but a single class A network number, while the
second block is a set of 16 contiguous class B network numbers, and
third block is a set of 256 contiguous class C network numbers.
An enterprise that decides to use IP addresses out of the address
space defined in this document can do so without any coordination
with IANA or an Internet registry. The address space can thus be
used by many enterprises. Addresses within this private address
space will only be unique within the enterprise, or the set of
enterprises which choose to cooperate over this space so they may
communicate with each other in their own private internet.
As before, any enterprise that needs globally unique address space is
required to obtain such addresses from an Internet registry. An
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enterprise that requests IP addresses for its external connectivity
will never be assigned addresses from the blocks defined above.
In order to use private address space, an enterprise needs to
determine which hosts do not need to have network layer connectivity
outside the enterprise in the foreseeable future and thus could be
classified as private. Such hosts will use the private address space
defined above. Private hosts can communicate with all other hosts
inside the enterprise, both public and private. However, they cannot
have IP connectivity to any host outside of the enterprise. While
not having external (outside of the enterprise) IP connectivity
private hosts can still have access to external services via
application layer relays.
All other hosts will be public and will use globally unique address
space assigned by an Internet Registry. Public hosts can communicate
with other hosts inside the enterprise both public and private and
can have IP connectivity to public hosts outside the enterprise.
Public hosts do not have connectivity to private hosts of other
enterprises.
Moving a host from private to public or vice versa involves a change
of IP address.
Because private addresses have no global meaning, routing information
about private networks shall not be propagated on inter-enterprise
links, and packets with private source or destination addresses
should not be forwarded across such links. Routers in networks not
using private address space, especially those of Internet service
providers, are expected to be configured to reject (filter out)
routing information about private networks. If such a router
receives such information the rejection shall not be treated as a
routing protocol error.
Indirect references to such addresses should be contained within the
enterprise. Prominent examples of such references are DNS Resource
Records and other information referring to internal private
addresses. In particular, Internet service providers should take
measures to prevent such leakage.
4. Advantages and Disadvantages of Using Private Address Space
The obvious advantage of using private address space for the Internet
at large is to conserve the globally unique address space by not
using it where global uniqueness is not required.
Enterprises themselves also enjoy a number of benefits from their
usage of private address space: They gain a lot of flexibility in
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network design by having more address space at their disposal than
they could obtain from the globally unique pool. This enables
operationally and administratively convenient addressing schemes as
well as easier growth paths.
For a variety of reasons the Internet has already encountered
situations where an enterprise that has not been connected to the
Internet had used IP address space for its hosts without getting this
space assigned from the IANA. In some cases this address space had
been already assigned to other enterprises. When such an enterprise
later connects to the Internet, it could potentially create very
serious problems, as IP routing cannot provide correct operations in
presence of ambiguous addressing. Using private address space
provides a safe choice for such enterprises, avoiding clashes once
outside connectivity is needed.
A major drawback to the use of private address space is that it may
actually reduce an enterprise's flexibility to access the Internet.
Once one commits to using a private address, one is committing to
renumber all or part of an enterprise, should one decide to route an
entire enterprise to the Internet. Usually the cost of renumbering
can be measured by counting the number of hosts that have to
transition from private to public. As was discussed earlier,
however, even if a network uses globally unique addresses, it may
still have to renumber. Although not the case today, it is likely
that renumbering may be necessary, regardless of whether private or
public address space is used.
The cost of renumbering may well be mitigated by development and
deployment of tools that facilitate renumbering, make use of Dynamic
Host Configuration Protocol (DHCP). When deciding whether to use
private addresses, we recommend that one consult computer and
software vendors about availability of such tools.
If we review the examples we list in Section 2, we note that banks
tend to merge, as do companies with firewalls. Therefore it may be
advisable to discuss with management the implications of private
networks.
5. Operational Considerations
A recommended strategy is to design the private part of the network
first and use private address space for all internal links. Then
plan public subnets at the locations needed and design the external
connectivity.
This design is not fixed permanently. If a number of hosts require
to change status later this can be accomplished by renumbering only
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the hosts involved and installing another physical subnet if
required.
If a suitable subnetting scheme can be designed and is supported by
the equipment concerned, it is advisable to use the 24-bit block of
private address space and make an addressing plan with a good growth
path. If subnetting is a problem, the block of 255 /24 prefixes can
be used.
One might be tempted to have both public and private addresses on the
same physical medium. While this is possible, there are pitfalls to
such a design. We advise caution when proceeding in this area.
Moving a single host between private and public status will involve a
change of address and in most cases physical connectivity. In
locations where such changes can be foreseen (machine rooms etc.) it
may be advisable to configure separate physical media for public and
private subnets to facilitate such changes.
Changing the status of all hosts on a whole (sub)network can be done
easily and without disruption for the enterprise network as a whole.
Consequently it is advisable to group hosts whose connectivity needs
might undergo similar changes in the future on their own subnets.
It is strongly recommended that routers which connect enterprises to
external networks are set up with appropriate packet and routing
filters at both ends of the link in order to prevent packet and
routing information leakage. An enterprise should also filter any
private networks from inbound routing information in order to protect
itself from ambiguous routing situations which can occur if routes to
the private address space point outside the enterprise.
It is possible for two sites who both coordinate their private
address space to communicate with each other over a public network.
To do so they must use some method of encapsulation at their borders
to a public network, thus keeping their private addresses private.
If two (or more) organizations follow the address allocation
specified in this document and then later wish to establish IP
connectivity with each other, then there is a risk that address
uniqueness would be violated. To minimize the risk it is strongly
recommended that an organization that decides to use addresses out of
the blocks specified in this document selects a random contiguous
sub-block(s) for its internal allocation.
A possible approach to avoid leaking of DNS RRs is to run two
nameservers, one external server authoritative for all globally
unique IP addresses of the enterprise and one internal nameserver
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authoritative for all IP addresses of the enterprise, both public and
private. In order to ensure consistency both these servers should be
configured from the same data of which the external nameserver only
receives a filtered version.
The resolvers on all internal hosts, both public and private, query
only the internal nameserver. The external server resolves queries
from resolvers outside the enterprise and is linked into the global
DNS. The internal server forwards all queries for information
outside the enterprise to the external nameserver, so all internal
hosts can access the global DNS. This ensures that information about
private hosts does not reach resolvers and nameservers outside the
enterprise.
6. References
[RFC1466] Gerich, E., "Guidelines for Management of IP Address
Space", RFC 1466, Merit Network, Inc., May 1993.
[RFC1518]
[RFC1519]
7. Security Considerations
While using private address space can improve security, it is not a
substitute for dedicated security measures.
8. Conclusion
With the described scheme many large enterprises will need only a
relatively small block of addresses from the globally unique IP
address space. The Internet at large benefits through conservation
of globally unique address space which will effectively lengthen the
lifetime of the IP address space. The enterprises benefit from the
increased flexibility provided by a relatively large private address
space. However, use of private addressing requires that an
organization renumber part or all of its enterprise network, as its
needs change over time.
9. Acknowledgments
We would like to thank Tony Bates (RIPE NCC), Jordan Becker (ANS),
Hans-Werner Braun (SDSC), Ross Callon (BayNetworks), John Curran
(NEARNET), Vince Fuller (Barrnet), Tony Li (cisco Systems), Anne Lord
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(RIPE NCC), Milo Medin (NSI), Marten Terpstra (RIPE NCC), Geza
Turchanyi (RIPE NCC), Christophe Wolfhugel (Pasteur Institute), Andy
Linton (connect.com.au), Brian Carpenter (CERN), Randy Bush (PSG),
Erik Fair (Apple Computer), Dave Crocker (XXX), Tom Kessler (SGI),
and Dave Piscitello (Core Competence) for their review and
constructive comments.
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10. Authors' Addresses
Yakov Rekhter
Cisco systems
170 West Tasman Drive
San Jose, CA, USA
Phone: +1 914 528 0090
Fax: +1 408 526-4952
EMail: yakov@cisco.com
Robert G Moskowitz
Chrysler Corporation
CIMS: 424-73-00
25999 Lawrence Ave
Center Line, MI 48015
Phone: +1 810 758 8212
Fax: +1 810 758 8173
EMail: rgm3@is.chrysler.com
Daniel Karrenberg
RIPE Network Coordination Centre
Kruislaan 409
1098 SJ Amsterdam, the Netherlands
Phone: +31 20 592 5065
Fax: +31 20 592 5090
EMail: Daniel.Karrenberg@ripe.net
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Geert Jan de Groot
RIPE Network Coordination Centre
Kruislaan 409
1098 SJ Amsterdam, the Netherlands
Phone: +31 20 592 5065
Fax: +31 20 592 5090
EMail: GeertJan.deGroot@ripe.net
Eliot Lear
Mail Stop 15-730
Silicon Graphics, Inc.
2011 N. Shoreline Blvd.
Mountain View, CA 94043-1389
Phone: +1 415 960 1980
Fax: +1 415 961 9584
EMail: lear@sgi.com
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