More On Address Classes

Example 1:

in a class A network, only values from 00000000 to 01111111 are possible. Of these 128 combinations, 126 are permitted (networks 0 and 127 are reserved for other purposes). Hence there can only ever be 126 class A networks, each of which can have a huge number of hosts.
 

Example 2:

ironbark's IP address is 149.144.21.60 This is a class B network (149.144.0.0), and ironbark's host ID within this network is 21.60. Note that when we write an IP address with all-zeroes in the host part, we are referring to the "network number" itself.
 

Example 3:

203.17.3.5 is a class C address. The network number is 203.17.3.0, and the host number is 5.