 |
This article is uncategorized. Please categorize this article to list it with similar articles. (December 2007) |
Route aggregation is a technique which is used to conserve the problem of address space exhaustion as well as to limit the amount of routing information being advertised. The term Classless Inter-Domain Routing is a route aggregation technique which was used in the early 90's to deal with these problems. From a conceptual point of view, CIDR will take a block of contiguous class C addresses and represent them with a pair: (network address, count) This pair shows the smallest network address in the block, and the count indicates the number of networks in the block. For example: (192.168.1.0, 4) = 192.168.1.0, 192.168.2.0, 192.168.3.0, 192.168.4.0. The pair represents a total of 4 network address. The advantage of this can be demonstrated if you look at the internet as a collection of ISP's which each own a contiguous block of IP network numbers. Any one of the ISP's routing tables must have an entry for each of it's own addresses, but needs only one entry for any other ISP's address's. This clearly reduces the size of routing tables. In practise, CIDR does not restrict network numbers to class C networks nor does it use a count to specify block sizes. Instead CIDR requires the block of addresses to be a power of 2, and it uses a bit mask to identify the size of the block. Example: An ISP is assigned a block of IP addresses by a RIR; for example they may receive the address range of 172.1.0.0 to 172.1.255.255. They could then assign subnets to each of their downstream providers i.e.: Customer A will have the range 172.1.1.0 to 172.1.1.255, Customer B would receive the range 172.1.2.0 to 172.1.2.255 and Customer C would receive the range 172.1.3.0 to 172.1.3.255 and so on. Instead of an entry for each of the subnets 172.1.1.x and 172.1.2.x etc, the ISP could aggregate the entire 172.1.x.x address range and essentially advertise the network 172.1.0.0/16 on the internet community, which would reduce the number of entries in the global routing table. Comer, Douglas E. (2006). Internetworking with TCP/IP, 5, Prentice Hall: Upper Saddle River, NJ. .
