In IPv4 or Internet Protocol version 4, subnet mask classes are used to specify the routing procedure. A subnet generally provides the flow of undisrupted network traffic between hosts that are to be isolated based on a network configuration. By following systematic procedure of organizing these hosts into logical groups, subnetting improves security and speed of the process. The subnet mask is like an IP address that contains 4 bytes and is also represented in the similar ‘dotted-decimal’ notation like that of a general IP address.

For instance, 255.255.255.0 is the network mask for 11111111 11111111 11111111 00000000. If this mask authorizes a subnet with a larger network, it is also called the subnet mask. This form of denoting network mask is only valid in IPv4.




Similar to that, IP subnet masks numbers divides the networks into subnets. The mask number stands for the number of ones in the binary that is 'masked" against the address. It does such so that it can ignore the last bits which are for the group of IP addresses stored in the masked addresses.

A subnet mask neither bears the complete characteristics of an IP address not is it completely independent from it. Actually, IP and subnet masks go hand in hand where the latter accompanies the former and the two works together. Applying a subnet mask into an IP address splits it into two parts:

1.       Extended Network Address
2.       Host address

Basically, the network address +1 or the host/broadcast address -1 is the headway to the Internet.

For a subnet mask to be valid, the leftmost bits must be set to 1.
For example:
00000000 00000000 00000000 00000000 , is an invalid subnet mask as the leftmost digits are set to zero. Thus the correct format for that will be the following:
11111111 11111111 11111111 11111111
       

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