GROW                                                           V. Fuller
Internet-Draft                                                     T. Li
Expires: October 16, 2005                                  Cisco Systems
                                                          April 14, 2005

 Classless Inter-Domain Routing (CIDR): The Internet Address Assignment
                          and Aggregation Plan

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on October 16, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This memo discusses the strategy for address assignment of the
   existing 32-bit IPv4 address space with a view toward conserving the
   address space and limiting the growth rate of global routing state.
   This document obsoletes the original CIDR spec [RFC1519], with
   changes made both to clarify the concepts it introduced and, after

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   more than twelve years, to update the Internet community on the
   results of deploying the technology described.

Table of Contents

   1.   History and Problem Description  . . . . . . . . . . . . . .   3
   2.   Classless addressing as a solution . . . . . . . . . . . . .   4
     2.1  Basic concept and prefix notation  . . . . . . . . . . . .   5
   3.   Address assignment and routing aggregation . . . . . . . . .   7
     3.1  Aggregation efficiency and limitations . . . . . . . . . .   7
     3.2  Distributed assignment of address space  . . . . . . . . .   9
   4.   Routing implementation considerations  . . . . . . . . . . .  10
     4.1  Rules for route advertisement  . . . . . . . . . . . . . .  10
     4.2  How the rules work . . . . . . . . . . . . . . . . . . . .  11
     4.3  A note on prefix filter formats  . . . . . . . . . . . . .  12
     4.4  Responsibility for and configuration of aggregation  . . .  12
     4.5  Route propagation and routing protocol considerations  . .  14
   5.   Example of new address assignments and routing . . . . . . .  14
     5.1  Address delegation . . . . . . . . . . . . . . . . . . . .  14
     5.2  Routing advertisements . . . . . . . . . . . . . . . . . .  16
   6.   Domain Name Service considerations . . . . . . . . . . . . .  17
   7.   Transition to a long term solution . . . . . . . . . . . . .  19
   8.   Analysis of CIDR's effect on global routing state  . . . . .  19
   9.   Conclusions and Recommendations  . . . . . . . . . . . . . .  21
   10.  Security Considerations  . . . . . . . . . . . . . . . . . .  21
   11.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  23
   12.  References . . . . . . . . . . . . . . . . . . . . . . . . .  23
     12.1   Normative References . . . . . . . . . . . . . . . . . .  23
     12.2   Informative References . . . . . . . . . . . . . . . . .  23
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  25
        Intellectual Property and Copyright Statements . . . . . . .  26

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1.  History and Problem Description

   What is now known as the Internet started as a research project in
   the 1970s to design and develop a set of protocols that could be used
   with many different network technologies to provide a seamless, end-
   to-end facility for interconnecting a diverse set of end systems.
   When determining how the 32-bit address space would be used, certain
   assumptions were made about the number of organizations to be
   connected, the number of end systems per organization, and total
   number of end systems on the network.  The end result was the
   establishment (see [RFC791]) of three classes  of networks: class A
   (most significant address bits '00'), with 128 possible networks each
   with 16777216 end systems (minus special bit values reserved for
   network/broadcast addresses); class B (MSB '10'), with 16384 possible
   networks each with 65536 end systems (less reserved values); and
   class C (MSB '110'), with 2097152 possible networks each with 254 end
   systems (256 bit combinations minus the reserved all-zeros and all-
   ones patterns).  The set of addresses with MSB '111' was reserved for
   future use; parts of this were eventually defined (MSB '1110') for
   use with IPv4 multicast and parts are still reserved as of the
   writing of this document.

   In the late 1980s, the expansion and commercialization of the former
   research network resulted in the connection of many new organizations
   to the rapidly-growing Internet and each new organization required an
   address assignment according to the class A/B/C addressing plan.  As
   demand for new network numbers, particularly in the class B space
   started to take on what appeared to be an exponential growth rate,
   some members of the operations and engineering community started to
   have concerns over the long-term scaling properties of the class
   A/B/C system and began thinking about how to modify network number
   assignment policy and routing protocols to better accommodate the
   growth.  In November, 1991, the IETF created the ROAD (Routing and
   Addressing) group to examine the situation.  This group met in
   January, 1992 and identified three major problems:

   1.  Exhaustion of the class B network address space.  One fundamental
       cause of this problem is the lack of a network class of a size
       which is appropriate for mid-sized organization; class C, with a
       maximum of 254 host addresses, is too small, while class B, which
       allows up to 65534 host addresses, is too large for most
       organizations but was the best fit available for use with

   2.  Growth of routing tables in Internet routers beyond the ability
       of current software, hardware, and people to effectively manage.

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   3.  Eventual exhaustion of the 32-bit IPv4 address space.

   It was clear that then-current rates of Internet growth would cause
   the first two problems to become critical some time between 1993 and
   1995.  Work already in progress on topological assignment of
   addressing for CLNS, which was presented to the community at the
   Boulder IETF in December of 1990, led to thoughts on how to re-
   structure the 32-bit IPv4 address space to increase its lifespan.
   Work in the ROAD group followed and eventually resulted in the
   publication of [RFC1338] and later [RFC1519].

   The design and deployment of CIDR was intended to solve these
   problems by providing a mechanism to slow the growth of global
   routing tables and to reduce the rate of consumption of IPv4 address
   space.  It did not and does not attempt to solve the third problem,
   which is of a more long-term nature, but instead endeavors to ease
   enough of the short to mid-term difficulties to allow the Internet to
   continue to function efficiently while progress is made on a longer-
   term solution.

   More historical background on this effort and on the ROAD group may
   be found in [RFC1380] and at [LWRD].

2.  Classless addressing as a solution

   The solution that the community created was to deprecate the Class
   A/B/C network address assignment system in favor of using
   "classless", hierarchical blocks of IP addresses (referred to as
   prefixes).  The assignment of prefixes is intended to roughly follow
   the underlying Internet topology so that aggregation can be used to
   facilitate scaling of the global routing system.  One implication of
   this strategy is that prefix assignment and aggregation is generally
   done according to provider-subscriber relationships, since that is
   how the Internet topology is determined.

   When originally proposed in [RFC1338] and [RFC1519], this addressing
   plan was intended to be a relatively short-term response, lasting
   approximately three to five years during which a more permanent
   addressing and routing architecture would be designed and
   implemented.  As can be inferred from the dates on the original
   documents, CIDR has far outlasted its anticipated lifespan and has
   become the mid-term solution to the problems described above.

   Coupled with address management strategies implemented by the
   Regional Internet Registries (see [NRO] for details), the deployment
   of CIDR-style addressing has also reduced the rate at which IPv4
   address space has been consumed, thus providing short-to-medium-term
   relief to problem #3 described above.

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   Note that, as defined, this plan neither requires nor assumes the re-
   assignment of those parts of the legacy "class C" space that are not
   amenable to aggregation (sometimes called "the swamp").  Doing so
   would somewhat reduce routing table sizes (current estimate is that
   "the swamp" contains approximately 15,000 entries) though at a
   significant renumbering cost.  Similarly, there is no hard
   requirement that any end site renumber when changing transit service
   provider but end sites are encouraged do so to eliminate the need for
   explicit advertisement of their prefixes into the global routing

2.1  Basic concept and prefix notation

   In the simplest sense, the change from Class A/B/C network numbers to
   classless prefixes is to make explicit which bits in a 32-bit IPv4
   address are interpreted as the network number (or prefix) associated
   with a site and which are the used to number individual end systems
   within the site.  In CIDR notation, a prefix is shown as a 4-octet
   quantity, just like a traditional IPv4 address or network number,
   followed by the "/" (slash) character, followed by a decimal value
   between 0 and 32 that describes the number of significant bits.

   For example, the legacy "class B" network, with an implied
   network mask of, is defined as the prefix,
   the "/16" indicating that the mask to extract the network portion of
   the prefix is a 32-bit value where the most significant 16 bits are
   ones and the least significant 16 bits are zeros.  Similarly, the
   legacy "class C" network number is defined as the prefix - the most significant 24 bits are ones and the least
   significant 8 bits are zeros.

   Using classless prefixes with explicit prefix lengths allows much
   more flexible matching of address space blocks to actual need.  Where
   formerly only three network sizes were available, prefixes may be
   defined to describe any power-of-two-sized block of between one and
   2^32 end system addresses.  In practice, the unallocated pool of
   addresses is administered by the Internet Assigned Numbers Authority
   ([IANA]).  The IANA makes allocations from this pool to Regional
   Internet Registries, as required.  These allocations are made in
   contiguous bit-aligned blocks of 2^24 addresses (a.k.a. /8 prefixes).
   The RIRs, in turn, allocate or assign smaller address blocks to Local
   Internet Registries (LIRs) or Internet Service Providers (ISPs).
   These entities may make direct use of the assignment (as would
   commonly be the case for an ISP) or may make further sub-allocations
   of addresses to their customers.  These RIR address assignments vary
   according to the needs of each ISP or LIR.  For example, a large ISP
   might be allocated an address block of 2^17 addresses (a /15 prefix)
   while a smaller ISP may be allocated an address block of 2^11

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   addresses (a /21 prefix).

   Note that the terms "allocate" and "assign" have specific meaning in
   the Internet address registry system; "allocate" refers to the
   delegation of a block of address space to an organization which is
   expected to perform further sub-delegations while "assign" is used
   for sites that directly use (i.e. number individual hosts) the block
   of addresses received.

   The following table provides a convenient short-cut to all of the
   CIDR prefix sizes, showing the number of addresses possible in each
   prefix and the number of prefixes of that size that may be numbered
   in the 32-bit IPv4 address space:

       notation       addrs/block      # blocks
       --------       -----------     ----------
       n.n.n.n/32               1     4294967296    "host route"
       n.n.n.x/31               2     2147483648    "[RFC3021] p2p link"
       n.n.n.x/30               4     1073741824
       n.n.n.x/29               8      536870912
       n.n.n.x/28              16      268435456
       n.n.n.x/27              32      134217728
       n.n.n.x/26              64       67108864
       n.n.n.x/25             128       33554432
       n.n.n.0/24             256       16777216    legacy "class C"
       n.n.x.0/23             512        8388608
       n.n.x.0/22            1024        4194304
       n.n.x.0/21            2048        2097152
       n.n.x.0/20            4096        1048576
       n.n.x.0/19            8192         524288
       n.n.x.0/18           16384         262144
       n.n.x.0/17           32768         131072
       n.n.0.0/16           65536          65536    legacy "class B"
       n.x.0.0/15          131072          32768
       n.x.0.0/14          262144          16384
       n.x.0.0/13          524288           8192
       n.x.0.0/12         1048576           4096
       n.x.0.0/11         2097152           2048
       n.x.0.0/10         4194304           1024
       n.x.0.0/9          8388608            512
       n.0.0.0/8         16777216            256    legacy "class A"
       x.0.0.0/7         33554432            128
       x.0.0.0/6         67108864             64
       x.0.0.0/5        134217728             32
       x.0.0.0/4        268435456             16
       x.0.0.0/3        536870912              8
       x.0.0.0/2       1073741824              4

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       x.0.0.0/1       2147483648              2       4294967296              1    "default route"

   n is an 8-bit, decimal octet value.

   x is a 1 to 7 bit value, base on the prefix length, shifted into the
   most significant bits of the octet and converted into decimal form;
   the least significant bits of the octet are zero.

   In practice, prefixes of length shorter than 8 are not allocated or
   assigned though routes to such short prefixes may exist in routing
   tables if or when aggressive aggregation is performed.  As of the
   writing of this document, no such routes are seen in the global
   routing system but operator error and other events have caused some
   of them (i.e. and to be observed in some
   networks at some times in the past.

3.  Address assignment and routing aggregation

   Classless addressing and routing was initially developed primarily to
   improve the scaling properties of routing on the global Internet.
   Because the scaling of routing is very tightly coupled to the way
   that addresses are used, deployment of CIDR had implications for the
   way in which addresses were assigned.

3.1  Aggregation efficiency and limitations

   The only commonly-understood method for reducing routing state on a
   packet-switched network is through aggregation of information.  For
   CIDR to succeed in reducing the size and growth rate of the global
   routing system, the IPv4 address assignment process needed to be
   changed to make possible the aggregation of routing information along
   topological lines.  Since, in general, the topology of the network is
   determined by the service providers who have built it, topologically-
   significant address assignments are necessarily service-provider

   Aggregation is simple for an end site which is connected to one
   service provider: it uses address space assigned by its service
   provider and that address space is a small piece of a larger block
   allocated to the service provider.  No explicit route is needed for
   the end site - the service provider advertises a single aggregate
   route for the larger block; this advertisement provides reachability
   and routeability for all of the customers numbered in the block.

   There are two, more complex, situations that reduce the effectiveness
   of aggregation:

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   o  An organization which is multi-homed.  Because a multi-homed
      organization must be advertised into the system by each of its
      service providers, it is often not feasible to aggregate its
      routing information into the address space of any one of those
      providers.  Note that the organization still may receive its
      address assignment out of a service provider's address space
      (which has other advantages), but a route to the organization's
      prefix must still be explicitly advertised by all of its service
      providers.  For this reason, the global routing cost for a multi-
      homed organization is generally the same as it was prior to the
      adoption of CIDR.

   o  An organization which changes service provider but does not
      renumber.  This has the effect of "punching a hole" in one of the
      original service provider's aggregated route advertisements.  CIDR
      handles this situation by requiring the newer service provider to
      advertise a specific advertisement for the re-homed organization;
      this advertisement is preferred over provider aggregates because
      it is a longer match.  To maintain efficiency of aggregation, it
      is recommended that an organization which changes service
      providers plan to eventually migrate its network into a an prefix
      assigned from its new provider's address space.  To this end, it
      is recommended that mechanisms to facilitate such migration, such
      as dynamic host address assignment using [RFC2131]) be deployed
      wherever possible, and that additional protocol work be done to
      develop improved technology for renumbering.

   Note that some aggregation efficiency gain can still be had for
   multi-homed sites (and, in general, for any site composed of
   multiple, logical IPv4 networks) - by allocating a contiguous power-
   of-two block address space to the site (as opposed to multiple,
   independent prefixes) the site's routing information may be
   aggregated into a single prefix.  Also, since the routing cost
   associated with assigning a multi-homed site out of a service
   provider's address space is no greater than the old method of
   sequential number assignment by a central authority, it makes sense
   to assign all end-site address space out of blocks allocated to
   service providers.

   It is also worthwhile to mention that since aggregation may occur at
   multiple levels in the system, it may still be possible to aggregate
   these anomalous routes at higher levels of whatever hierarchy may be
   present.  For example, if a site is multi-homed to two relatively
   small providers that both obtain connectivity and address space from
   the same large provider, then aggregation by the large provider of
   routes from the smaller networks will include all routes to the
   multi-homed site.  The feasibility of this sort of second-level
   aggregation depends on whether topological hierarchy exists between a

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   site, its directly-connected providers, and other providers to which
   they are connected; it may be practical in some regions of the global
   Internet but not in others.

   Note: in the discussion and examples which follow, prefix notation is
   used to represent routing destinations.  This is used for
   illustration only and does not require that routing protocols use
   this representation in their updates.

3.2  Distributed assignment of address space

   In the early days of the Internet, IPv4 address space assignment was
   performed by the central Network Information Center (NIC).  Class
   A/B/C network numbers were assigned in essentially arbitrary order,
   roughly according to the size of the organizations that requested
   them.  All assignments were recorded centrally and no attempt was
   made to assign network numbers in a manner that would allow routing

   When CIDR was originally deployed, the central assignment authority
   continued to exist but changed its procedures to assign large blocks
   of "Class C" network numbers to each service provider.  Each service
   provider, in turn, assigned bitmask-oriented subsets of the
   provider's address space to each customer.  This worked reasonably
   well as long as the number of service providers was relatively small
   and relatively constant but did not scale well as the number of
   service providers grew at a rapid rate.

   As the Internet started to expand rapidly in the 1990s, it became
   clear that a single, centralized address assignment authority was
   problematic.  This function began being de-centralized when address
   space assignment for European Internet sites was delegated in bit-
   aligned blocks of 16777216 addresses (what CIDR would later define as
   a /8) to the RIPE NCC ([RIPE]), effectively making it the first of
   the RIRs.  Since then, address assignment has been formally
   distributed as a hierarchical function with IANA, the RIRs, and the
   service providers.  Removing the bottleneck of a single organization
   having responsibility for the global Internet address space greatly
   improved the efficiency and response time for new assignments.

   Hierarchical delegation of addresses in this manner implies that
   sites with addresses assigned out of a given service provider are,
   for routing purposes, part of that service provider and will be
   routed via its infrastructure.  This implies that routing information
   about multi-homed organizations, i.e., organizations connected to
   more than one network service provider, will still need to be known
   by higher levels in the hierarchy.

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   Some of these issues are discussed at greater length in [RFC1518].

4.  Routing implementation considerations

   With the change from classful network numbers to classless prefixes,
   it is not possible to infer the network mask from the initial bit
   pattern of an IPv4 address.  This has implications for how routing
   information is stored and propagated.  Network masks or prefix
   lengths must be explicitly carried in routing protocols.  Interior
   routing protocols such as OSPF [RFC2178], IS-IS [RFC1195], RIPv2
   [RFC2453], and Cisco EIGRP, and the BGP4 exterior routing protocol
   [RFC1771] all support this functionality, having been developed or
   modified as part of the deployment of classless inter-domain routing
   during the 1990s.

   Older interior routing protocols, such as RIP [RFC1058], HELLO, and
   Cisco IGRP, and older exterior routing protocols, such as EGP
   [RFC904], do not support explicit carriage of prefix length/mask and
   thus cannot be effectively used on the Internet in other than very
   limited, stub configurations.  While their use may be appropriate in
   simple, legacy end-site configurations, they are considered obsolete
   and should NOT be used in transit networks connected to the global

   Similarly, routing and forwarding tables in layer-3 network equipment
   must be organized to store both prefix and prefix length or mask.
   Equipment which organizes its routing/forwarding information
   according to legacy class A/B/C network/subnet conventions cannot be
   expected to work correctly on networks connected to the global
   Internet; use of such equipment is not recommended.  Fortunately,
   very little such equipment is in use today.

4.1  Rules for route advertisement

   1.  Routing to all destinations must be done on a longest-match basis
       only.  This implies that destinations which are multi-homed
       relative to a routing domain must always be explicitly announced
       into that routing domain - they cannot be summarized (this makes
       intuitive sense - if a network is multi-homed, all of its paths
       into a routing domain which is "higher" in the hierarchy of
       networks must be known to the "higher" network).

   2.  A router which generates an aggregate route for multiple, more-
       specific routes must discard packets which match the aggregate
       route but not any of the more-specific routes.  In other words,
       the "next hop" for the aggregate route should be the null
       destination.  This is necessary to prevent forwarding loops when
       some addresses covered by the aggregate are not reachable.

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   Note that during failures, partial routing of traffic to a site which
   takes its address space from one service provider but which is
   actually reachable only through another (i.e., the case of a site
   which has changed service providers) may occur because such traffic
   will be forwarded along the path advertised by the aggregated route.
   Rule #2 will prevent packet mis-delivery by causing such traffic to
   be discarded by the advertiser of the aggregated route, but the
   output of "traceroute" and other similar tools will suggest that a
   problem exists within that network rather than in the network which
   is no longer advertising the more-specific prefix.  This may be
   confusing to those trying to diagnose connectivity problems; see the
   example in Section 5.2 for details.  A solution to this perceived
   "problem" is beyond the scope of this document - it lies with better
   education of the user/operator community, not in routing technology.

   An implementation following these rules should also be generalized,
   so that an arbitrary network number and mask are accepted for all
   routing destinations.  The only outstanding constraint is that the
   mask must be left contiguous.  Note that the degenerate route to
   prefix is used as a default route and MUST be accepted by
   all implementations.  Further, to protect against accidental
   advertisements of this route via the inter-domain protocol, this
   route should only be advertised when a router is explicitly
   configured to do so - never as a non-configured, "default" option.

4.2  How the rules work

   Rule #1 guarantees that the routing algorithm used is consistent
   across implementations and consistent with other routing protocols,
   such as OSPF.  Multi-homed networks are always explicitly advertised
   by every service provider through which they are routed even if they
   are a specific subset of one service provider's aggregate (if they
   are not, they clearly must be explicitly advertised).  It may seem as
   if the "primary" service provider could advertise the multi-homed
   site implicitly as part of its aggregate, but the assumption that
   longest-match routing is always done causes this not to work.

   Rule #2 guarantees that no routing loops form due to aggregation.
   Consider a site that has been assigned 192.168.64/19 by its "parent"
   provider that has  The "parent" network will
   advertise to the "child" network.  If the "child"
   network were to lose internal connectivity to (which
   is part of its aggregate), traffic from the "parent" to the to the
   "child" destined for will follow the "child's"
   advertised route.  When that traffic gets to the "child", however,
   the mid-level *must not* follow the route back up to
   the "parent", since that would result in a forwarding loop.  Rule #2
   says that the "child" may not follow a less-specific route for a

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   destination which matches one of its own aggregated routes
   (typically, this is implemented by installing a "discard" or "null"
   route for all aggregated prefixes which one network advertises to
   another).  Note that handling of the "default" route ( is a
   special case of this rule - a network must not follow the default to
   destinations which are part of one of it's aggregated advertisements.

4.3  A note on prefix filter formats

   Systems which process route announcements must be able to verify that
   information which they receive is acceptable according to policy
   rules.  Implementations which filter route advertisements must allow
   masks or prefix lengths in filter elements.  Thus, filter elements
   which formerly were specified as:


   now look something like:


   This is merely making explicit the network mask which was implied by
   the class A/B/C classification of network numbers.  It is also useful
   to enhance filtering capability to allow the match of a prefix and
   all more-specific prefixes with the same bit pattern; fortunately,
   this functionality has been implemented by most vendors of equipment
   used on the Internet.

4.4  Responsibility for and configuration of aggregation

   Under normal circumstances, a routing domain (or "Autonomous System")
   which has been allocated or assigned a set of prefixes has sole
   responsibility for aggregation of those prefixes.  In the usual case,
   the AS will install configuration in one or more of its routers to
   generate aggregate routes based on more-specific routes known to its
   internal routing system; these aggregate routes are advertised into
   the global routing system by the border routers for the routing
   domain.  The more-specific internal routes which overlap with the
   aggregate routes should not be advertised globally.  In some cases,
   an AS may wish to delegate aggregation responsibility to another AS

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   (for example, a customer may wish for its service provider to
   generate aggregated routing information on its behalf); in such
   cases, aggregation is performed by a router in the second AS based on
   the routes that it receives from the first combined with configured
   policy information describing how those routes should be aggregated.

   It should be mentioned that one provider may choose to perform
   aggregation on the routes it receives from another without explicit
   agreement; this is termed "proxy aggregation".  This can be a useful
   tool for reducing the amount of routing state that an AS must carry
   and propagate to its customers and neighbors, proxy aggregation can
   also create inconsistencies in global routing state.  Consider what
   happens if both AS 2 and 3 receive routes from AS 1 but AS 2 performs
   proxy aggregation while AS 3 does not.  Other AS's which receive
   transit routing information from both AS 2 and AS 3 will see an
   inconsistent view of the routing information originated by AS 1.
   This may cause an unexpected shift of traffic toward AS 1 through AS
   3 for AS 3's customers and any others receiving transit routes from
   AS 3.  Because proxy aggregation can cause unanticipated consequences
   for parts of the Internet that have no relationship with either the
   source of the aggregated routes or the party providing aggregation,
   it should be used with extreme caution.

   Configuration of the routes to be combined into aggregates is an
   implementation of routing policy and does require some manually-
   maintained information.  As an addition to the information that must
   be maintained for a set of routeable prefixes, aggregation
   configuration is typically just a line or two defining the range of
   the block of IPv4 addresses to aggregate.  A site performing its own
   aggregation is doing so for address blocks that it has been assigned;
   a site performing aggregation on behalf of another knows this
   information based on an agreement to delegate aggregation.  Assuming
   a best common practice for network administrators to exchange lists
   of prefixes to accept from one and other, configuration of
   aggregation information does not introduce significant additional
   administrative overhead.

   The generation of an aggregate route is usually specified either
   statically or in response to learning an active dynamic route for a
   prefix contained within the aggregate route.  If such dynamic
   aggregate route advertisement is done, care should be taken that
   routes are not excessively added or withdrawn (known as "route
   flapping"); in general, a dynamic aggregate route advertisement is
   added when at least one component of the aggregate becomes reachable
   and it is withdrawn only when all components become unreachable.
   Properly configured, aggregated routes are more stable than non-
   aggregated routes and thus improve global routing stability.

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   Implementation note: aggregation of the "Class D" (multicast) address
   space is beyond the scope of this document.

4.5  Route propagation and routing protocol considerations

   Prior to the original deployment of CIDR, common practice was to
   propagate routes learned via exterior routing protocols (i.e.  EGP or
   BGP) through a site's interior routing protocol (typically, OSPF,
   IS-IS, or RIP).  This was done to ensure that consistent and correct
   exit points were chosen for traffic destined to a destination learned
   through those protocols.  Four evolutionary effects -- the advent of
   CIDR, explosive growth of global routing state, widespread adoption
   of BGP4, and a requirement to propagate full path information -- have
   combined to deprecate that practice.  To ensure proper path
   propagation and prevent inter-AS routing inconsistency (BGP4's loop
   detection/prevention mechanism requires full path propagation),
   transit networks must use internal BGP (iBGP) for carrying routes
   learned from other providers both within and through their networks.

5.  Example of new address assignments and routing

5.1  Address delegation

   Consider the block of 524288 (2^19) addresses beginning with and ending with allocated to a single network
   provider, "PA".  This is equivalent in size to a block of 2048 legacy
   "class C" network numbers (or /24s).  A classless route to this block
   would be described as with mask of the prefix

   Assume this service provider connects six sites in the following
   order (significant because it demonstrates how temporary "holes" may
   form in the service provider's address space):

   o  "C1" requiring fewer than 2048 addresses (/21 or 8 x /24)

   o  "C2" requiring fewer than 4096 addresses (/20 or 16 x /24)

   o  "C3" requiring fewer than 1024 addresses (/22 or 4 x /24)

   o  "C4" requiring fewer than 1024 addresses (/22 or 4 x /24)

   o  "C5" requiring fewer than 512 addresses (/23 or 2 x /24)

   o  "C6" requiring fewer than 512 addresses (/23 or 2 x /24)

   In all cases, the number of IPv4 addresses "required" by each site is
   assumed to allow for significant growth.  The service provider

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   delegates its address space as follows:

   o  C1: assign 10.24.0 through 10.24.7.  This block of networks is
      described by the route (mask

   o  C2: assign 10.24.16 through 10.24.31.  This block is described by
      the route (mask

   o  C3: assign 10.24.8 through 10.24.11.  This block is described by
      the route (mask

   o  C4: assign 10.24.12 through 10.24.15.  This block is described by
      the route (mask

   o  C5: assign 10.24.32 and 10.24.33.  This block is described by the
      route (mask

   o  C6: assign 10.24.34 and 10.24.35.  This block is described by the
      route (mask

   These six sites should be represented as six prefixes of varying size
   within the provider IGP.  If, for some reason, the provider were to
   use an obsolete IGP that doesn't support classless routing or
   variable-length subnets, then then explicit routes all /24s will have
   to be carried.

   To make this example more realistic, assume that C4 and C5 are multi-
   homed through some other service provider, "PB".  Further assume the
   existence of a site "C7" which was originally connected to "RB" but
   has moved to "PA".  For this reason, it has a block of network
   numbers which are assigned out "PB"'s block of (the next) 2048 x /24.

   o  C7: assign 10.32.0 through 10.32.15.  This block is described by
      the route (mask

   For the multi-homed sites, assume that C4 is advertised as primary
   via "RA" and secondary via "RB"; C5 is primary via "RB" and secondary
   via "RA".  In addition, assume that "RA" and "RB" are both connected
   to the same transit service provider "BB".

   Graphically, this topology looks something like this:

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Internet-Draft            CIDR Address Strategy               April 2005 --         __10.32.0.0 -
   C1:        \       /  C7:
                            \     /
                             +----+                              +----+ -  |    |                              |    |
   C2:       \ |    |  _10.24.12.0 -  |    |
                            \|    | / C4:        \ |    |
                             |    |/                            \|    | -| PA |\                             | PB |
   C3:          |    | \__10.24.32.0 -|    |
                             |    |    C5:         |    |
                             |    |                              |    | -|    |                              |    |
   C6:         |    |                              |    |
                             +----+                              +----+
                               ||                                  ||
   routing advertisements:     ||                                  ||
                               ||                                  ||
  (C4)  ||     (C4)  ||
  (C7)   ||     (C5)  ||
  (PA)   ||     (PB)   ||
                               ||                                  ||
                               VV                                  VV
                            +---------- BACKBONE NETWORK BB ----------+

5.2  Routing advertisements

   To follow rule #1, PA will need to advertise the block of addresses
   that it was given and C7.  Since C4 is multi-homed and primary
   through PA, it must also be advertised.  C5 is multi-homed and
   primary through PB.  In principal (and in the example above), it need
   not be advertised since longest match by PB will automatically select
   PB as primary and the advertisement of PA's aggregate will be used as
   a secondary.  In actual practice, C5 will normally be advertised via
   both providers.

   Advertisements from "PA" to "BB" will be:

 primary    (advertises C4)
 primary     (advertises C7)
 primary     (advertises remainder of PA)

   For PB, the advertisements must also include C4 and C5 as well as
   it's block of addresses.

   Advertisements from "PB" to "BB" will be:

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 secondary  (advertises C4)
 primary    (advertises C5)
 primary     (advertises remainder of RB)

   To illustrate the problem diagnosis issue mentioned in Section 4.1,
   consider what happens if PA loses connectivity to C7 (the site which
   is assigned out of PB's space).  In a stateful protocol, PA will
   announce to BB that has become unreachable.  Now, when
   BB flushes this information out of its routing table, any future
   traffic sent through it for this destination will be forwarded to PB
   (where it will be dropped according to Rule #2) by virtue of PB's
   less specific match  While this does not cause an
   operational problem (C7 is unreachable in any case), it does create
   some extra traffic across "BB" (and may also prove confusing to
   someone trying to debug the outage with "traceroute").  A mechanism
   to cache such unreachable state might be nice but is beyond the scope
   of this document.

6.  Domain Name Service considerations

   One aspect of Internet services which was notably affected by the
   move to CIDR was the mechanism used for address-to-name translation:
   the IN-ADDR.ARPA zone of the domain system.  Because this zone is
   delegated on octet boundaries only, the move to an address assignment
   plan which uses bitmask-oriented addressing caused some increase in
   work for those who maintain parts of the IN-ADDR.ARPA zone.

   As described above, the IN-ADDR.ARPA zone is necessarily organized
   along octet boundaries.  Prior to the adoption of CIDR, IN-ADDR.ARPA
   was also constrained such that delegations were only permitted along
   legacy, class A/B/C network number boundaries.  This created a
   difficult situation for more flexible, CIDR prefixes.  Consider a
   hypothetical large network provider named "BigNet" which has been
   allocated the block through (the CIDR prefix  Under the old delegation policies, the top-level IN-
   ADDR.ARPA domain servers would need to have 1024 entries of the form:

       0.4.10.IN-ADDR.ARPA.   IN      NS      NS1.BIG.NET.

       1.4.10.IN-ADDR.ARPA.   IN      NS      NS1.BIG.NET.


       255.7.10.IN-ADDR.ARPA. IN      NS      NS1.BIG.NET.

   By revising the policy as described above, this was reduced to four
   delegation records:

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       4.10.IN-ADDR.ARPA.     IN      NS      NS1.BIG.NET.

       5.10.IN-ADDR.ARPA.     IN      NS      NS1.BIG.NET.

       6.10.IN-ADDR.ARPA.     IN      NS      NS1.BIG.NET.

       7.10.IN-ADDR.ARPA.     IN      NS      NS1.BIG.NET.

   The provider then maintains further delegations of naming authority
   for each individual /24 which it assigns, rather than having each
   registered separately.  Note that due to the way the DNS is designed,
   it is still possible for the top-level IN-ADDR.ARPA name servers to
   maintain the delegation information for individual networks for which
   the provider is unwilling or unable to do so.  The example above
   illustrates only the records for a single name server.  In the normal
   case, there are usually several name servers for each domain, thus
   the size of the examples will double or triple in the common cases.

   For BIG.NET to assign a blocks smaller than /24 to its customers, it
   can similarly delegate DNS authority for those addresses.  For
   example, if it were to assign to its customer
   CUSTONE.COM and to its customer CUSTTWO.COM, it could
   add the following records to delegate DNS for the addresses to those
   customers:    IN    NS    NS1.CUSTONE.COM.    IN    NS    NS1.CUSTONE.COM.

              ....   IN    NS    NS1.CUSTONE.COM   IN    NS    NS1.CUSTTWO.COM   IN    NS    NS1.CUSTTWO.COM

              ....   IN    NS    NS1.CUSTTWO.COM

   And if BIG.NET also assigned to CUSTTHREE.COM but this
   customer did not want to run its own DNS, BIG.NET could provide that
   service for the customer by installing the appropriate PTR records:

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Internet-Draft            CIDR Address Strategy               April 2005   IN    PTR   NET.CUSTTHREE.COM.   IN    PTR   HOST1.CUSTTHREE.COM.   IN    PTR   HOST2.CUSTTHREE.COM.   IN    PTR   BCAST.CUSTTHREE.COM.

   See [RFC2317] for a much more detailed discussion of DNS delegation
   with classless addressing.

7.  Transition to a long term solution

   CIDR was designed to be a short-term solution to the problems of
   routing state and address depletion on the IPv4 Internet.  It does
   not change the fundamental Internet routing or addressing
   architectures.  It is not expected to affect any plans for transition
   to a more long-term solution except, perhaps, by delaying the urgency
   of developing such a solution.

8.  Analysis of CIDR's effect on global routing state

   When CIDR was first proposed in the early 1990s, the original authors
   made some observations about the growth rate of global routing state
   and offered projections on how CIDR deployment would, hopefully,
   reduce what appeared to be exponential growth to a more sustainable
   rate.  Since that deployment, an ongoing effort, called "The CIDR
   Report" [CRPT] has attempted to quantify and track that growth rate.
   What follows is a brief summary of the CIDR report as of March, 2005,
   with an attempt to explain the various patterns of and change in
   growth rate that have occurred since measurements of the size of
   global routing state began in 1988.

   Examining the graph of "Active BGP Table Entries" [CBGP] there appear
   to be several different growth trends with distinct inflection points
   reflecting changes in policy and practice.  The trends and events
   which are believed to have caused them were:

   1.  Exponential growth at the far left of the graph.  This represents
       the period of early expansion and commercialization of the former
       research network, from the late 1980s through approximately 1994.
       The major driver for this growth was a lack of aggregation
       capability for transit providers, and the widespread use of
       legacy Class C allocations for end sites.  Each time a new site
       was connected to the global Internet, one or more new routing
       entries were generated.

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   2.  Acceleration of the exponential trend in late 1993 and early 1994
       as CIDR supernet" blocks were first assigned by the NIC and
       routed as separate legacy class-C networks by service provider.

   3.  A sharp drop in 1994 as BGP4 deployment by providers allowed
       aggregation of the "supernet" blocks.  Note that the periods of
       largest declines in the number of routing table entries typically
       correspond to the weeks following each meeting of the IETF CIDR
       Deployment Working Group.

   4.  Roughly linear growth from mid-1994 to early 1999 as CIDR-based
       address assignments were made and aggregated routes added
       throughout the network.

   5.  A new period of exponential growth again from early 1999 until
       2001 as the "high-tech bubble" fueled both rapid expansion of
       Internet as well as a large increase in more-specific route
       advertisements for multi-homing and traffic engineering.

   6.  Flattening of growth through 2001 caused by a combination of the
       "dot-com bust", which caused many organizations to cease
       operations, and the "CIDR police" [CPOL] work aimed at improving
       aggregation efficiency.

   7.  Roughly linear growth through 2002 and 2003.  This most likely
       represents a resumption of the "normal" growth rate observed
       before the "bubble" as well as an end to the "CIDR Police"

   8.  A more recent trend of exponential growth beginning in 2004.  The
       best explanation would seem to be an improvement of the global
       economy driving increased expansion of the Internet and the
       continued absence of the "CIDR Police" effort, which previously
       served as an educational tool for new providers to improve
       aggregation efficiency.  There have also been some cases where
       service providers have deliberately de-aggregated prefixes in an
       attempt to mitigate security problems caused by conflicting route
       advertisements (see Section 10).  While this behavior may solve
       the short-term problems seen by such providers, it is
       fundamentally non-scalable and quite detrimental to the community
       as a whole.  In addition, there appear to be many providers
       advertising both their allocated prefixes and all of the /24
       components of them, probably due to a lack of consistent current
       information about recommended routing configuration.

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9.  Conclusions and Recommendations

   In 1992, when CIDR was first developed, there were serious problems
   facing the continued growth of the Internet.  Growth in routing state
   complexity, and the rapid increase in consumption of address space
   made it appear that one or both problems would preclude continued
   growth of the Internet within a few short years.

   Deployment of CIDR, in combination with BGP4's support for carrying
   classless prefix routes, alleviated the short-term crisis.  It was
   only through a concerted effort by both the equipment manufacturers
   and the provider community that this was achieved.  The threat (and,
   perhaps in some cases, actual implementation of) charging networks
   for advertising prefixes may have offered an additional incentive to
   share the address space, and hence the associated costs of
   advertising routes to service providers.

   The IPv4 routing system architecture carries topology information
   based on aggregate address advertisements and a collection of more-
   specific advertisements that are associated with traffic engineering,
   multi-homing and local configuration.  As of March, 2005, the base
   aggregate address load in the routing system has some 75,000 entries.
   Approximately 85,000 additional entries are more specific entries of
   this base "root" collection.  There is reason to believe that many of
   these additional entries are exist to solve problems of regional or
   even local scope and should not need to be globally propagated.

   An obvious question to ask is whether CIDR can continue to be a
   viable approach to keeping global routing state growth and address
   space depletion at sustainable rates.  Recent measurements indicate
   that exponential growth has resumed but further analysis suggests
   that this trend can be mitigated by a more active effort to educate
   service providers on efficient aggregation strategies and proper
   equipment configuration.  Looking farther forward, there is a clear
   need for better multi-homing technology that does not require global
   routing state for each site and for methods of performing traffic
   load balancing that do not require adding even more state.  Without
   such developments and in the absence of major architectural change,
   aggregation is the only tool available for making routing scale in
   the global Internet.

10.  Security Considerations

   The introduction of routing protocols which support classless
   prefixes and a move to a forwarding model that mandates that more-
   specific (longest-match) routes be preferred when they overlap with
   routes to less-specific prefixes introduces at least two security

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      Traffic can be hijacked by advertising a prefix for a given
      destination that is more specific than the aggregate that is
      normally advertised for that destination.  For example, assume a
      popular end system with address that is connected
      to a service provider that advertises  A
      malicious network operator interested in intercepting traffic for
      this site might advertise, or at least attempt to advertise, into the global routing system.  Because this
      prefix is more-specific than the "normal" prefix, traffic will be
      diverted away from the legitimate end system and to the network
      owned by the malicious operator.  Prior to the advent of CIDR, it
      was possible to induce traffic from some parts of the network to
      follow a false advertisement that exactly matched a particular
      network number; CIDR makes this problem somewhat worse, since
      longest-match routing generally causes all traffic to prefer more-
      specific routes over less-specific routes.  The remedy for the
      CIDR-based attack, though, is the same as for a pre-CIDR-based
      attack: establishment of trust relationships between providers,
      coupled with and strong route policy filters at provider borders.
      Unfortunately, the implementation of such filters is difficult in
      the highly de-centralized Internet.  As a workaround, many
      providers do implement generic filters that set upper bounds,
      derived from RIR guidelines for the sizes of blocks that they
      allocate, on the lengths of prefixes that are accepted from other
      providers.  It is worth noting that "spammers" have been observed
      using this sort of attack to temporarily hijack address space in
      order to hide the origin of the traffic ("spam" email messages)
      that they generate.

      Denial-of-service attacks can be launched against many parts of
      the Internet infrastructure by advertising a large number of
      routes into the system.  Such an attack is intended to cause
      router failures by overflowing routing and forwarding tables.  A
      good example of a non-malicious incident which caused this sort of
      failure was the infamous "AS 7007" event [7007] where a router
      mis-configuration by an operator caused a huge number of invalid
      routes to be propagated through the global routing system.  Again,
      this sort of attack is not really new with CIDR; using legacy
      class A/B/C routes, it was possible to advertise a maximum of
      16843008 unique network numbers into the global routing system, a
      number which is sufficient to cause problems for even the most
      modern routing equipment made in 2005.  What is different is that
      the moderate complexity of correctly configuring routers in the
      presence of CIDR does tend to make accidental "attacks" of this
      sort more likely.  Measures to prevent this sort of attack are
      much the same as those described above for the hijacking, with the
      addition that best common practice is to also configure a
      reasonable maximum number of prefixes that a border router will

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      accept from its neighbors.

      Note that this is not intended to be an exhaustive analysis of the
      sorts of attacks that CIDR makes easier; a more comprehensive
      analysis of security vulnerabilities in the global routing system
      is beyond the scope of this document.

11.  Acknowledgments

   The authors wish to express appreciation to the other original
   authors of RFC1519 (Kannan Varadhan, Jessica Yu), to the ROAD group
   with whom many of the ideas behind CIDR were inspired and developed,
   and to the early reviewers of this re-spun version of the document
   (Barry Greene, Geoff Huston, Danny McPherson, Dave Meyer, Eliot Lear,
   Bill Norton, Ted Seely, Philip Smith) whose comments, corrections,
   and suggestions were invaluable.

12.  References

12.1  Normative References

   [RFC791]  Postel, J., "Internet Protocol", RFC 791, September 1981.

12.2  Informative References

   [RFC1338]  Fuller, V., Li, T., Varadhan, K., and J. Yu,
              "Supernetting: an Address Assignment and Aggregation
              Strategy", RFC 1338, June 1992.

   [RFC1519]  Fuller, V., Li, T., Varadhan, K., and J. Yu, "Classless
              Inter-Domain Routing: an Address Assignment and
              Aggregation Strategy", RFC 1519, September 1993.

   [IANA]     "Internet Assigned Numbers Authority",

   [NRO]      "Number Resource Organization", <>.

   [RFC1380]  Gross, P. and P. Almquist, "IESG Deliberations on Routing
              and Addressing", RFC 1380, November 1992.

   [LWRD]     "The Long and Winding Road",

   [RFC2178]  Moy, J., "The OSPF Specification Version 2", RFC 2178,
              July 1997.

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   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, December 1990.

   [RFC2453]  Malkin, G., "RIP Version 2", RFC 2453, November 1998.

   [RFC1771]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
              (BGP-4)", RFC 1771, March 1995.

   [RFC1058]  Hedrick, C., "Routing Information Protocol", RFC 1058,
              June 1988.

   [RFC904]   Mills, D., "Exterior Gateway Protocol formal
              specification", RFC 904, April 1984.

   [RFC3021]  Retana, A., White, R., Fuller, V., and D. McPherson,
              "Using 31-Bit Prefixes on IPv4 Point-to-Point Links",
              RFC 3021, December 2000.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RIPE]     "RIPE Network Coordination Centre", <>.

   [RFC1518]  Rekhter, Y. and T. Li, "An Architecture for IP Address
              Allocation with CIDR", RFC 1518, September 1993.

   [RFC2317]  Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
              ADDR.ARPA delegation", RFC 2317, March 1998.

   [CRPT]     "The CIDR Report", <>.

   [CBGP]     "Graph: Active BGP Table Entries, 1988 to Present",

   [CPOL]     "CIDR Police - Please Pull Over and Show Us Your BGP",

   [7007]     "NANOG mailing list discussion of the "AS 7007" incident",

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Authors' Addresses

   Vince Fuller
   170 W. Tasman Drive
   San Jose, CA  95134


   Tony Li
   170 W. Tasman Drive
   San Jose, CA  95134


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