Determination of IP address class is looking at its first octet, i.e. the first numbers showing the values of each of the four bytes in decimal form. In General, any IP address is a concatenation of two logical parts:

network number;
- node number in the network.

The first bits of the IP address and determine what portion of this address shows the network number and which is the node number in the network. Also by default, each address class uses its own subnet mask.
Class network And have addresses starting with 0 to 126, and the subnet mask Thus, the number 127 is designed for special purposes, and the number 0 is not used. An example of such ' -, where octet is the number 10.
The value of the first octet in the range 128 to 191 whether the network to a class b subnet Mask of these networks is An example of such address may serve, in which the first octet is the number 172.
If the IP address starts with digits in the range from 192 to 223, it belongs to the class S. These addresses use a subnet mask of As an example of ' class can serve as, the first octet which is 192.
A separate group address or multicast starting with digits 1110 belongs to the class D. the Purpose of the address class D address of a packet implies receipt of this packet by all nodes with the same IP address. Subnet mask address class-D -
Another class of IP addresses are reserved for future use, is a class E. the First digits of these addresses is the sequence 11110, and the subnet mask used by the address class E, acts
Please note that in certain cases, the subnet mask value can not meet the requirements of some organizations, and may be reassigned.