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CIDR: route aggregation calculation done the easy way

Using class C networks 200.200.0.0/24  through 200.200.7.0/24, define a CIDR mask that will permit a router to list all 8 networks as a single entry. This will speed up routing table lookups on the router.

The third byte is the octet that is changing. Convert it to binary for each of the networks:

200.200.0.0 = 00000000

200.200.1.0 = 00000001

skip a few networks

200.200.6.0 = 00000110

200.200.7.0 = 00000111

Which bits are not changing at all in the third octet? The first 5 of the 8.

What is the mask of the first two bytes (these bytes don't change either)? /16

Cumulatively, how many bits do not change? 16 + 5 = 21

What is the first address of this address range? 200.200.0.0

Therefore, the final address and mask as a single router entry that represents 8 networks numbered 200.200.0.0 thru 200.200.7.0, is: 200.200.0.0/21