Subnet Mask - Networking
To facilitate intra-network routing, a single IP network can be divided into many subnets by using some of the most significant bits of the host address portion of the IP address as a subnet ID.
For example, Network 188.8.131.52 has 16 bits assigned as the network ID (specifically 129.5, in dotted decimal, which is 10000001.00000101 in binary) as it is a Class B address (the first number is between 128 and 192).
This leaves the lower 16 bits as the host address.
Using a Subnet Bit Mask of 255.255.255.0 (which in binary is 11111111.11111111.11111111.00000000), specifies that the upper 24 bits (those that are set to a 1) are the network plus subnet address--that is, the 16 bits that we already expected plus the upper 8 bits of the host address. Therefore the network 184.108.40.206 (in this example) would consist of up to 254 subnets (220.127.116.11 through 18.104.22.168) of up to 254 hosts each.
This is useful for subdividing networks, for example, to reduce the number of stations that must receive broadcasts.