Cisco Routers for the Desperate, 2nd Edition

Practical Netmasks

The end result of netmasks being in binary is that IP addresses are issued in blocks that are multiples of two. If you have 5 bits to play with, you'll have 32 (2 x 2 x 2 x 2 x 2 = 32) IP addresses on your network, and your netmask will be a /27 (32 – 5 = 27). If you have 8 bits for your hosts, you have (2⁸) or 256 IP addresses. If you're told that you have 55 IP addresses, you're either sharing a network with other people or you need to learn what your network administrator is drinking and get him to share.

UNUSABLE IP ADDRESSES

The first and last address in any block is unusable. You cannot assign a machine on a network with a netmask of 255.255.255.0 an IP ending in either .0 or .255.

It's not uncommon to see a host IP with ...