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Internet addresses are allocated by the InterNIC (http://www.internic.net ). Each address classes has a default subnet mask. You can identify the class of an IP address by looking at its first octet. 

  • Class A networks use a default subnet mask of 255.0.0.0 and have 0-127 as their first octet. The address 10.52.36.11 is a class A address. Its first octet is 10, between 1 and 126, inclusive.
  • Class B networks use a default subnet mask of 255.255.0.0 and have first octet between 128-191, inclusive. The address 172.16.52.63 is a class B address, its first octet is 172.
  • Class C networks use a default subnet mask of 255.255.255.0 and have first octet between 192-223. The address 192.168.123.132 is a class C address, its first octet being 192.

Classes D and E exist, but are not generally used by end users.

There are three Private IP addresses. Private means that these will not be forwarded on the internet as a whole.

Class A - 10.x.x.x - 16,777,216 addresses 
Class B - 172.16.x.x - 172.31.x.x - 16 blocks of 65536 addresses 
Class C - 192.168.x.x - 255 blocks of 256 addresses. 

Subnet or Host/Network Discriminator

Using a subnet mask of 255.255.255.192, your 192.168.123.0 network then becomes the four networks 192.168.123.0, 192.168.123.64, 192.168.123.128 and 192.168.123.192. These four networks would have as valid host addresses:

 
  192.168.123.1-62
   192.168.123.65-126
   192.168.123.129-190
   192.168.123.193-254

Remember with /26 or 255.255.255.192 , host addresses with all ones ( BROADCAST ) or all zeros ( NETWORK ADDRESS ) are invalid, so you cannot use addresses with the last octet of 0, 63, 64, 127, 128, 191, 192, or 255.

You can see how this works by looking at two host addresses, 192.168.123.71 and 192.168.123.133. If you used the default Class C subnet mask of 255.255.255.0, both addresses are on the 192.168.123.0 network. However, if you use the subnet mask of 255.255.255.192, they are on different networks; 192.168.123.71 is on the 192.168.123.64 network, 192.168.123.133 is on the 192.168.123.128 network.

http://www.tcpipguide.com/free/t_IPAddressClassABandCNetworkandHostCapacities.htm