Network Working Group Internet Engineering Steering Group
Request for Comments: 1517 R. Hinden, Editor
Category: Standards Track September 1993
Applicability Statement for the Implementation of
Classless Inter-Domain Routing (CIDR)
Status of this Memo
This RFC specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.
As the Internet has evolved and grown in recent years, it has become
clear that it will soon face several serious scaling problems. These
- Exhaustion of the class-B network address space. One
fundamental cause of this problem is the lack of a network
class of a size that is appropriate for a mid-sized
organization. Class-C, with a maximum of 254 host addresses, is
too small, while class-B, which allows up to 65534 addresses,
is too large to be densely populated. The result is inefficient
utilization of class-B network numbers.
- Routing information overload. The size and rate of growth of the
routing tables in Internet routers is beyond the ability of
current software (and people) to effectively manage.
- Eventual exhaustion of IP network numbers.
It has become clear that the first two of these problems are likely
to become critical in the near term. Classless Inter-Domain Routing
(CIDR) ttempts to deal with these problems by defining a mechanism to
slow the growth of routing tables and reduce the need to allocate new
IP network numbers. It 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-
The IESG, after a thorough discussion in the IETF, in June 1992
selected CIDR as the solution for the short term routing table
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RFC 1517 CIDR Applicability Statement September 1993
explosion problem .
The CIDR architecture is described in the following documents:
- "An Architecture for IP Address Allocation with CIDR" 
- "Classless Inter-Domain Routing (CIDR): An Address Assignment
and Aggregation Strategy" 
The first of these documents presents the overall architecture of
CIDR; the second describes the specific address allocation scheme to
In addition to these two documents, "Guidelines for Management of IP
Address Space"  provides specific recommendations for assigning IP
addresses that are consistent with  and , and "Status of CIDR
Deployment in the Internet"  describes the timetable for deploying
 in the Internet. Both  and  should be viewed as
supporting, rather than defining, documents.
In addition to the documents mentioned above, CIDR requires that
inter-domain routing protocols be capable of handling reachability
information that is expressed solely in terms of IP address prefixes.
While several inter-domain routing protocols are capable of
supporting such functionality, this Applicability Statement does not
mandate the use of a particular one.
Although Internet routing domains are not required to use routing
protocols capable of propagating CIDR routes, the topology such
routing domains can support will be somewhat limited. In particular,
the non-CIDR-capable parts of the Internet will need to default
towards the CIDR-capable parts of the Internet for routes which have
been aggregated to non-network boundaries.
The CIDR architecture is applicable to any group of connected domains
that supports IP version 4  . CIDR does not require all of the
domains in the Internet to be converted to use CIDR. It assumes that
some of the existing domains in the Internet will never be able to
convert. Despite this, CIDR will still provide connectivity to such
places, although the optimality of routes to these places may be
This Applicability Statement requires Internet domains providing
backbone and/or transit service to fully implement CIDR in order to
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RFC 1517 CIDR Applicability Statement September 1993
ensure that the growth of the resources required by routers to
provide Internet-wide connectivity will be significantly slower than
the growth of the number of assigned networks.
This Applicability Statement strongly recommends that all non-
backbone/transit Internet domains also implement CIDR because it will
reduce the amount of routing information inside of these domains.
Individual domains are free to choose whatever inter-domain and
intra-domain routing architectures best meet their requirements.
Specifically, this Applicability Statement does not prevent a domain
or a group of domains from using addressing schemes which do not
conform to CIDR. Subject to the available resources in routers, CIDR
should be able to co-exist with other addressing schemes without
adversely impacting overall connectivity.
 Gross, P., and P. Almquist, "IESG Deliberations on Routing and
Addressing", RFC 1380, IESG Chair, IESG Internet AD, November
 Rekhter, Y., and T. Li, "An Architecture for IP Address
Allocation with CIDR", RFC 1518, T.J. Watson Research Center, IBM
Corp., cisco Systems, September 1993.
 Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
Domain Routing (CIDR): An Address Assignment and Aggregation
Strategy", RFC 1519, BARRNet, cisco, Merit, and OARnet, September
 Gerich, E., "Guidelines for Management of IP Address Space", RFC
1466, Merit, May 1993.
 Topolcic, C., "Status of CIDR Deployment in the Internet", RFC
1467, CNRI, August 1993.
 Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", STD 5, RFC 791, USC/Information Sciences
Institute, September 1981.
 Braden, R., Editor, "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, IETF, October 1989.