The IP Address (Part 2):

Address Mask, Subnet Mask, or Network Mask?

These three terms, address mask, subnet mask, and network mask, are used interchangeably to refer to the same thing—the bit mask that is used to determine the structure of an address. Because the ifconfig command uses the keyword netmask for the argument that defines the bit mask on the command line, the ifconfig documentation refers to this value as the network mask. Therefore, the term network mask is widely used by system administrators.

All IP addresses have an associated bit mask because it is needed to implement classless IP addresses. If your organization purchased an official block of addresses from an outside agency such as ARIN, you received an official network number and an address mask to go with it. This address/mask pair defines the total address space available to your organization. Sometimes, an organization will subdivide its official address space in order to create additional networks within that address structure. These additional networks are used to simplify management and routing, and are created by increasing the number of network bits in the address mask. Traditionally, network administrators call this subnetting, and sometimes they refer to the address mask as a subnet mask.

From the point of view of the individual network server, it makes no difference. The mask is defined in the same way on the ifconfig command, regardless of the name it is given—address mask, subnet mask, and network mask are all the same thing.

Even when the network you're working with uses an IP address that conforms to the class rules, don't allow the network mask to default to the class value. Always define the network mask on the command line. Addressing is too important to leave to chance; make sure you're in control of it. For the same reason, it is useful to define the broadcast address.