Subnet Mask

A 32-bit number expressed as four octets used to identify and separate the network portion and host portion of a 32 bit IP address. For example, a subnet mask of 255.0.0.0 identifes the first octet (255) as the network portion and the last three octets (0.0.0) as the host portion for a Class A IP address. This subnet mask for the Class A IP address (255.0.0.0) designates a relatively larger number of hosts when compared with the subnet mask for the Class C IP address (255.255.255.0) which designates a relatively larger number of networks and a smaller number of hosts.