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  • Sigitas Steponavicius thinks this is interesting:

The maximum number of network ID bits is 30. You have to leave at least two bits for the host ID portion of the address to allow for at least two hosts. If you use all 32 bits for the network ID, that leaves no bits for the host ID. Obviously, that won’t work. Leaving just one bit for the host ID won’t work, either, because a host ID of all ones is reserved for a broadcast address, and all zeros refers to the network itself. Thus, if you use 31 bits for the network ID and leave only 1 for the host ID, host ID 1 would be used for the broadcast address, and host ID 0 would be the network itself, leaving no room for actual hosts. That’s why the maximum network ID size is 30 bits.

From

Cover of Networking All-in-One For Dummies

Note

This is wrong - it can be 31 bits. http://www.ietf.org/rfc/rfc3021.txt