The terms netmask and subnet mask are often used interchangeably. However, subnet masks are used primarily in network configurations, while netmasks typically refer to classes of IP addresses. They are used to define a range of IP addresses that can be used by an ISP or other organization.
There are three standard classes of IP addresses – A, B, and C – which have the following netmasks:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
A Class A netmask defines a range of IP addresses in which the first three digit section is the same, but the other sections may each contain any number between 0 and 255. Class B addresses have the same first two sections, but the numbers in the second two sections can be different. Class C addresses have the same first three sections and only the last section can have different numbers. Therefore, a Class C IP address range may contain up to 256 addresses.
Technically, a netmask is a 32-bit value used to divide sections of IP addresses. While a class C netmask is commonly written "255.255.255.0," it may also be defined as 11111111.11111111.11111111.00000000. This binary representation reveals the 32 bits that make up the netmask (4 sections of 8 bits each). It also shows the way the netmask "masks" the IP addresses it contains. The sections with all 1's are predefined and cannot be changed, while the section with all 0's can be any number between 0 and 255.