Applicability Statement for the Implementation of Classless Inter-Domain Routing (CIDR)
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Applicability Statement for the Implementation of
Classless Inter-Domain Routing (CIDR)
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
"working draft" or "work in progress."
Please check the 1id-abstracts.txt listing contained in the
internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net,
nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the
current status of any Internet Draft.
This memo is an draft IESG standards track Applicability Statement for
the Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Official
Internet Protocol Standards" for the standardization state and status
of this specification. 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) attempts 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-term solution.
The IESG, after a thorough discussion in the IETF, in June 1992
selected CIDR as the solution for the short term routing table
explosion problem .
2 Components of the Architecture
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 be used.
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 "Schedule for Address
Space Management Guidelines"  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. The inter-domain protocols which
meets this requirement is:
Border Gateway Protocol version 4 
Inter-Domain Routing Protocol for IP 
Inter-Domain routing protocols which do not meet these requirements
Border Gateway Protocol version 3 
Exterior Gateway Protocol 
While CIDR does not require intra-domain routing protocols to also be
CIDR capable, it highly recommends that intra-domain routing protocols
Although CIDR does not require that intra-domain routing protocols, as
well as inter-domain routing protocols, be capable of supporting CIDR,
the benefits of implementing CIDR will be greater if this is the case.
If this is not done, then the CIDR route aggregation will need to be
undone inside of a routing domain. The CIDR capable intra-domain
routing protocols are:
Open Shortest Path Routing Protocol 
RIP Version 2 
The Intra-Domain routing protocol which is not CIDR capable is:
RIP Version 1 
3 Applicability of CIDR
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. On the
contrary, 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 impacted.
This Applicability Statement requires Internet domains providing
backbone and/or transit service to fully implement CIDR in order to
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.
3. SECURITY CONSIDERATIONS
Security issues are not discussed in this memo.
4. CONTACT INFORMATION
Robert M. Hinden
2550 Garcia Ave, MS MTV5-44
Mt. View, CA 94043
Phone: (415) 336-2082
Fax: (415) 336-6015
 Gross, P., Almquist, P., "IESG Deliberations on Routing and
Addressing", RFC1380, November 1992
 Rekhter, Y., Li, T., "An Architecture for IP Address Allocation
with CIDR" (currently an internet-draft)
 Fuller, V., Li, T., Yu, J., and Varadhan, K., "Classless Inter-
Domain Routing (CIDR): An Address Assignment and Aggregation
Strategy" (revision of RFC 1338)
 Gerich, E., "Guidelines for Management of IP Address Space",
RFC1366, October 1992
 Topolcic, C., "Schedule for address space management guidelines",
RFC 1367, October 1992 (the IESG has expressed an interest in
seeing this schedule revised to reflect the entire Internet; it is
 Rekhter, Y., Li, T., "A Border Gateway Protocol 4 (BGP-4)",
IETF Working Paper.
 Hares, S., "IDRP for IP", Internet Draft,
 Lougheed, K., Rekhter, Y., "A Border Gateway Protocol 3
(BGP-3)", RFC 1267, October 1991.
 Rosen, E.C., "Exterior Gateway Protocol EGP", RFC 827, October
 Moy, J., "OSPF Version 2", RFC 1247, Proteon, Inc., July
 Callon, R. "Use of OSI IS-IS for Routing in TCP/IP and Dual
Environments", RFC1195, December 1990.
 Malkin, G. "RIP Version 2 Carrying Additional Information",
RFC 1388, January 1993.
 Hedrick, C. "Routing Information Protocol", RFC 1058, June
 Postel, J.B. "Internet Protocol", RFC 791, September 1981.
 Braden, R., Editor, "Requirements for Internet Hosts -- Communication
Layers", IETF, STD 3, RFC 1122, October 1989.
 Almquist, P., Editor, "Requirements for IP Routers", Work in