Netmask and subnet mask are often used as if they’re the same thing, but subnet masks are mostly used in network configurations, while netmasks often refer to classes of IP addresses. They’re used to define a range of IP addresses that an ISP or other organization can use.
Three standard classes of IP addresses exist – A, B, and C – and they have the following netmasks:
- Class A: 255.0.0.0
- Class B: 255.255.0.0
- Class C: 255.255.255.0
Class A defines a range of IP addresses in which the first three-digit section is the same, but the other sections can each contain a number from 0 to 255. With Class B the same first two sections have the same number and with class C its the first three. Which means that a Class C IP address range may have up to 256 addresses.
A netmask is a 32-bit value that divides IP addresses into sections. While a class C netmask is often written like “255.255.255.0,” it may also be defined as 11111111.11111111.11111111.00000000. Showing it in binary demonstrates that there are 32 bits (4 sections of 8 bits each) which make up the netmask. It also shows how the netmask gets its name—”masking” the IP addresses within it. The sections with all 1s are pre-set and so can’t be changed, while the section with all 0s allows any number between 0 and 255.