3Com 3c17300 Implementation Guide - Page 91

Subnets and Subnet Masks 93 As shown in this example, the 32 bits of an IP address and subnet mask are usually written using an integer shorthand. This notation translates four consecutive 8-bit groups (octets) into four integers that range from 0 through 255. The subnet mask in the example is written as 255.255.255.0. Traditionally, subnet masks were applied to octets in their entirety. However, one octet in the subnet mask can be further subdivided so that part of the octet indicates an extension of the network number, and the rest of the same octet indicates the host number, as shown in Figure 30. Figure 30 Extending the Network Prefix Take the IP address IP address Network Subnet and Host Apply the subnet mask Subnet mask 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 Result = subnet/host boundary Nentewtwoorkr Ssuubbnnet Host Using the Class B IP address from Figure 29 (158.101.230.52), the subnet mask is 255.255.255.240. The number that includes both the Class B natural network mask (255.255) and the subnet mask (255.240) is sometimes called the extended network prefix. Continuing with the previous example, the subnetwork part of the mask uses 12 bits, and the host part uses the remaining 4 bits. Because the octets are actually binary numbers, the number of subnetworks that are possible with this mask is 4,096 (212), and the number of hosts that are possible in each subnetwork is 16 (24). Subnet Mask Numbering An alternate method to represent the subnet mask numbers is based on the number of bits that signify the network portion of the mask. Many Internet Service Providers (ISPs) now use this notation to denote the subnet mask. See Table 9.