F I G U R E   3 2 .   R e d u c e d   S i z e   o f   I n t e r n e t   R o u t i n g   Ta b l e s

Figure 32 illustrates how the allocation described in the previous CIDR

example helps reduce the size of the Internet routing tables. Assume

that a portion of the ISP's address block (200.25.16.0/20) has been allo 

cated as described in the previous example: 

  Organization A aggregates eight /24s into a single advertisement

(200.25.16.0/21)

  Organization B aggregates four /24s into a single advertisement

(200.25.24.0/22)

  Organization C aggregates two /24s into a single advertisement

(200.25.28.0/23)

  Organization D aggregates two /24s into a single advertisement

(200.25.30.0/23) 

Then the ISP can inject the 256 /24s in its allocation into the Internet

with a single advertisement 200.25.0.0/16. 

Note that route aggregation by means of BGP 4 (the protocol that allows

CIDR aggregation) is not automatic. The network engineers must config 

ure each router to perform the required aggregation. The successful

deployment of CIDR allows the number of individual networks on the

Internet to expand while minimizing the number of routes in the Inter 

net routing tables. 

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