Route aggregation: Difference between revisions

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-'''Route aggregation''' is a technique that 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 (CIDR) is a route aggregation technique that 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 as a pair of numerical terms:
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-(network address, count)
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-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 addresses. The advantage of this can be demonstrated if you look at the Internet as a collection of ISPs, each of which own a contiguous block of IP network numbers. Any one of the ISP's routing tables must have an entry for each of its own addresses, but needs only one entry for any other ISP's addresses. This clearly reduces the size of routing tables. In practice, 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 address]]es by a regional Internet registry (RIR); for example they may receive the address range of 172.1.0.0 to 172.1.255.255.  They could then assign [[subnet]]s to each of their downstream providers, e.g.: ''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 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.
-[[Category:Routing protocols]]