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Unformatted text preview: t is also called supernetting and is described in RFC 1518
[Rekhter and Li 1993] and RFC 1519 [Fuller et al. 1993], with a overview in [Ford, Rekhter,
and Braun 1993]. CIDR has the Internet Architecture Board's blessing [Huitema 1993]. RFC
1467 [Topolcic 1993] summarizes the state of deployment of CIDR in the Internet.
The basic concept in CIDR is to allocate multiple IP addresses in a way that allows
summarization into a smaller number of routing table entries. For example, if a single site is
allocated 16 class C addresses, and those 16 are allocated so that they can be summarized,
then all 16 can be referenced through a single routing table entry on the Internet. Also, if
eight different sites are connected to the same Internet service provider through the same
connection point into the Internet, and if the eight sites are allocated eight different IP
addresses that can be summarized, then only a single routing table entry need be used on the
Internet for all eight sites.
Three features are needed to allow this summarization to take place.
1. Multiple IP addresses to be summarized together for routing must share the same highorder bits of their addresses.
2. The routing tables and routing algorithms must be extended to base their routing
decisions on a 32-bit IP address and a 32-bit mask.
3. The routing protocols being used must be extended to carry the 32-bit mask in
addition to the 32-bit address. OSPF (Section 10.6) and RIP-2 (Section 10.5) are both
capable of carrying the 32-bit mask, as is the proposed BGP Version 4.
As an example, RFC 1466 [Gerich 1993] recommends that new class C addresses in Europe
be in the range 18.104.22.168 through 22.214.171.124. In hexadecimal these addresses are from
0xc2000000 through 0xc3ffffff. This represents 65536 different class C network IDs,
but they all share the same high-order 7 bits. In countries other than Europe a single routing
table entry with an IP address of 0xc2000000 and a 32-bit mask of 0xfe000000
(254.0.0.0) could be used to route all of these 65536 class C network IDs to a single point.
Subsequent bits of the class C address (that is, the bits following 194 or 195) can also be
allocated hierarchically, perhaps by country or by service provider, to allow additional
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12