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