F I G U R E   3 5 .   I S P   # 2 's   M o re   S p e c i f i c   R o u t e   i n t o   t h e   I n t e r n e t

The best strategy is for Organization A to retain ownership of its

address space and have ISP #2 advertise an  exception  (more specific)

route into the Internet. The exception route allows all traffic for

200.25.0.0/16 to be sent to ISP #1, with the exception of the traffic to

200.25.16.0/21. This routing is accomplished by having ISP #2 adver 

tise, in addition to its own 199.30.0.0/16 block, a route for

200.25.16.0/21. Refer to Figure 35. 

Using the longest match forwarding algorithm, Internet routers will

route traffic addressed to host 200.25.17.25 to ISP #2, which will in

turn route the traffic to Organization A. Clearly, the introduction of a

large number of exception routes can reduce the effectiveness of the

CIDR deployment and eventually cause Internet routing tables to begin

exploding again. 

Additional Practice with CIDR 

Appendix E provides exercises using CIDR. 

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