CIDR Reducing Routing Table Explosion
Given the somewhat hierarchical structure of the Internet, you can see how the structure lends itself to an address summarization scheme. At the top layers, large blocks of contiguous Class C addresses are assigned by the Internet Assigned Numbers Authority (IANA) to the various addressing authorities around the globe, known as the regional IP registries. Currently, there are three regional registries. The regional registry for North and South America, the Caribbean, and sub-Saharan Africa is the American Registry for Internet Numbers (ARIN). ARIN also is responsible for assigning addresses to the global network service providers. The regional registry for Europe, the Middle East, northern Africa, and parts of Asia (the area of the former Soviet Union) is the Resèaux IP Européens (RIPE). The regional registry for the rest of Asia and the Pacific nations is the Asia Pacific Network Information Center (APNIC).
NOTE ARIN was spun off of the InterNIC (run by Network Solutions, Inc.) in 1997 to separate the management of IP addresses and domain names.
Table 2-2 shows the original scheme for assigning Class C addresses to the regions these registries serve, although some of the allocations are now outdated. As Example 2-8 demonstrates, the blocks labeled "Others" are now being assigned. The regional registries, in turn, assign portions of these blocks to the large service providers or to local IP registries. Generally, the blocks assigned at this level are no smaller than 32 contiguous Class C addresses (and are usually larger). Concentric Network has been assigned the block 207.155.128.0/17, for example, which includes the equivalent of 128 contiguous Class C addresses (see Example 2-8).
Table 2-2 CIDR Address Allocation by Geographic Region
Table 2-2 shows the original scheme for assigning Class C addresses to the regions these registries serve, although some of the allocations are now outdated. As Example 2-8 demonstrates, the blocks labeled "Others" are now being assigned. The regional registries, in turn, assign portions of these blocks to the large service providers or to local IP registries. Generally, the blocks assigned at this level are no smaller than 32 contiguous Class C addresses (and are usually larger). Concentric Network has been assigned the block 207.155.128.0/17, for example, which includes the equivalent of 128 contiguous Class C addresses (see Example 2-8).
Table 2-2 CIDR Address Allocation by Geographic Region
|
Region |
Address Range |
|
Multiregional |
192.0.0.0-193.255.255.255 |
|
Europe |
194.0.0.0-195.255.255.255 |
|
Others |
196.0.0.0-197.255.255.255 |
continues continues
|
Region |
Address Range |
|
North America |
198.0.0.0-199.255.255.255 |
|
Central/South |
200.0.0.0-201.255.255.255 |
|
America |
|
|
Pacific Rim |
202.0.0.0-203.255.255.255 |
|
Others |
204.0.0.0-205.255.255.255 |
Others 206.0.0.0-207.255.255.255
Example 2-8 When a WHOIS Is Performed on the Address 207.155.128.5from Example 2-7, the Address Is Shown as Part of a/17 CIDR Block Assigned to Concentric Network
--- performing WHOIS on "207.155.128.5", please wait... --- contacting host whois.arin.net --- smart query on "207.155.128"
Concentric Research Corp. (NETBLK-CONCENTRIC-CIDR) 10590 N. Tantau Ave. Cupertino, CA 95014
Netname: CONCENTRIC-CIDR
Netblock: 207.155.128.0 - 207.155.255.255
Maintainer: CRC
Coordinator:
DNS and IP ADMIN (DIA-ORG-ARIN) [email protected] (408) 342-2800 Fax- (408) 342-2810
Domain System inverse mapping provided by:
NAMESERVER3.CONCENTRIC.NET 206.173.119.72
NAMESERVER2.CONCENTRIC.NET 207.155.184.72
NAMESERVER1.CONCENTRIC.NET 207.155.183.73
NAMESERVER.CONCENTRIC.NET 207.155.183.72
Record last updated on 13-Feb-97.
Database last updated on 29-Jan-99 16:12:40 EDT.
The service providers receiving these blocks assign them in smaller blocks to their subscribers. If those subscribers are themselves ISPs, they can again break their blocks into smaller blocks. The obvious advantage of assigning these blocks of Class C addresses, called CIDR blocks, comes when the blocks are summarized back up the hierarchy. For more information on how addresses are assigned throughout the Internet, see RFC 2050 (www.isi.edu/in-notes/rfc2050.txt).
To illustrate, suppose Concentric Network assigns to one of its subscribers a portion of its 207.155.128.0/17 block, consisting of 207.155.144.0/20. If that subscriber is an ISP, it may assign a portion of that block, say 207.155.148.0/22, to one of its own subscribers. That subscriber advertises its 122 (read "slash twenty-two") block back to its ISP. That ISP in turn summarizes all of its subscribers to Concentric Network with the single aggregate 207.155.144.0/20, and Concentric Network summarizes its subscribers into the NAPs to which it is attached with the single aggregate 207.155.128.0/17.
The advertisement of a single aggregate to the higher-level domain is obviously preferable to advertising possibly hundreds of individual addresses. But an equally important benefit is the stability such a scheme adds to the Internet. If the state of a network in a low-level domain changes, that change is felt only up to the first aggregation point and no further.
Table 2-3 shows the different sizes of CIDR blocks, their equivalent size in Class C networks, and the number of hosts each block can represent.
Table 2-3 CIDR Block Sizes
Table 2-3 shows the different sizes of CIDR blocks, their equivalent size in Class C networks, and the number of hosts each block can represent.
Table 2-3 CIDR Block Sizes
|
CIDR Block Prefix Size |
Number of Equivalent Class C Addresses |
Number of Possible Host Addresses |
|
/24 |
1 |
254 |
|
/23 |
2 |
510 |
|
122 |
4 |
1022 |
|
¡21 |
8 |
2046 |
|
¡20 |
16 |
4094 |
|
/19 |
32 |
8190 |
|
/18 |
64 |
16,382 |
|
111 |
128 |
32,766 |
|
/16 |
256 |
65,534 |
|
/15 |
512 |
131,070 |
|
/14 |
1024 |
262,142 |
|
/13 |
2048 |
524,286 |
Continue reading here: Difficulties with CIDR
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