Large-Scale IP Network Solutions (CCIE Professional Development)

Variable-Length Subnet Masking

With eight-bit masking, there are 254 subnets and 254 hosts per subnet. This model works well on transit broadcast networks, in which a large number of hosts share a common media. As shown in Figure 2-6, the serial line needs only two addresses to assign an address with a subnet mask of /24. Therefore, leaving space for 254 hosts is a waste of address space.

Address waste is a serious problem in today's networks. Obtaining an IP address is difficult because of constant growth and increasing numbers of users. Aggressive effort is required to spare address space from being used inappropriately. This issue will be discussed further in the next section, “Classless Interdomain Routing.”

For this reason, you should perform ...