The class of address can be determined easily by examining the first octet of the address and
mapping that value to a class range in the following table. In an IP address of 172.31.1.2, for
example, the first octet is 172. Because 172 falls between 128 and 191, 172.31.1.2 is a Class B
address. Figure 30-5 summarizes the range of possible values for the first octet of each address class.