Chapter 10 Dynamic routing protocols
10.8 CIDR: Classless Inter-Domain Routing
In Chapter 3, we pointed out the lack of class B addresses, so now multiple network sites can only use class C network numbers, rather than using a single class B network number. Although these Class C addresses assigned to solve a problem (lack of class B addresses), but it brings up another problem: Each Class C network requires a routing table entry. Classless Inter-Domain Routing (CIDR) is a method of preventing the I nternet expanded routing table, which is also called Ultra Network (supernetting). In 1518 the RFC [Rekher
and of Li 1993] and RFC 1519 [Fuller et al. 1993 ] It is described in, and [Ford, Rekhter, and Braun 1 9 9 3] It is a review. CIDR has an Internet Architecture Board's blessing [Huitema 1993 ]. RF C1467 [Topolcic 1993] on the I nternet development status in CIDR were summary.
CIDR basic idea is to adopt a way of allocating a plurality of IP addresses, it is possible to sum a number of entries in the routing table (Summarization) into a smaller number. For example, if the assigned class C addresses 16 to a single site, can be used in a way that the sum of the allocated address 16, so that, all 16 address can refer to a single routing table entry on the I nternet.
Meanwhile, if there are eight different sites through the same connection point is connected to the same I nternet I nternet service provider, and eight different IP addresses assigned to the eight sites may be the sum, then, for eight sites, on the I nternet, only a single routing table entry.
To use this sum must satisfy the following three characteristics:
- For routing is to be performed on the sum of a plurality of IP addresses, the IP addresses must have the same upper address bits.
- Routing tables and routing algorithms must be extended to make routing decisions based on the 32 bit IP address and 32 bit mask.
- It must be extended routing protocol in addition to the 32 bit address, but also a 32 bit mask. The OSPF (Section 106.) And RIP - 2 (. Section 105) are capable of carrying 32 bit mask BGP version 4 raised.
For example, RFC 1466 [Gerich 1993] recommend new class C address range in Europe is 194. 0.0. 0 ~ 195. 255. 255. 255. To 16 hexadecimal, the scope of these addresses is 0 xc 2 0 0 0 0 0 0 ~ 0 xc 3 ffffff. It represents 65,536 different Class C network number, but the 7 bit high address thereof is the same. In countries outside of Europe, the IP address can be used as 0 xc 2 0 0 0 0 0 0, and 32 bit 0xfe000000 (254.0.0.0) for the mask to a single routing table entry for all of these 65 536 C- class network numbers to a single routing point. Class C addresses behind each bit (i.e., 194 or 195 bits each later) can also be assigned levels, such as a country or service provider assigns, to allow the use of routers between Europe except that 32 bit other high bit 7 bit mask outside summarized.
CIDR also uses a technique that best match the longest match is always: i.e., 32 bit mask, which has a maximum. We continue to use the example used in the previous paragraph, a European service provider may use a different service providers in Europe and other access points. If the address assigned to the provider from the 194 group. 0.1 6.0 to 194.0.31.255 (16 class C network number), the IP address may be the network routing table entry is 194 . 0.1 6.0 mask is 255.255.240.0 (0xfffff 0 0 0). 194 destined. 0.2 2.1 IP packets simultaneously table entries and other entries European Class C addresses match with the route. However, since the mask 255. 255. 240 than 254. 0.0. 0 "longer", and is therefore in the routing table entry having longer mask.
"No type" means that the current routing decisions are based on the mask operation of the entire 32 bit IP address, regardless of its IP address is a Class A, Class B or Class C, makes no difference.
CIDR was originally aimed at new Class C addresses proposed. This change will make the I nternet routing table growth rate slow down, but for the existing routing is not any help. This is a short-term solution. As a long-term solution, if applied to all the CIDR IP addresses, and according to all continents and border service providers IP address already exists redistribution (and all existing hosts re-addressing!), Then [Ford, Rekhter, and Braun 1993] claimed that now includes 10,000 net
network routing table entries will be reduced to only 200 table entries.
