The Internet Mask

The Internet community originally identified three classes of organization. Companies or organizations were deemed to fall into one of three sizes or classes: small organizations into Class C, medium organizations into Class B, and large organizations into Class A. Actually, five classes of addresses are used in the Internet. The other two classes represent multicast (Class D) and experimental addresses (Class E). Routing protocols and videoconferencing increasingly use Class D addresses.

A router identifies the class of address by looking at the first few bits of the 32-bit address. When looking at the address in a decimal format, the number in the first octet reveals the class of address. This is known as the first octet rule.

Table 3-2 shows how the classes are broken up.

Table 3-2 The Classes of Addresses

Table 3-2 shows how the classes are broken up.

Table 3-2 The Classes of Addresses

Class of Address

First Octet

Number of Hosts That Address Could Represent on One Network

Class A address

001 to 126

Could represent 16.77 million hosts on one network

Class B address

128 to 191

Could represent 65,000 hosts on one network

Class C address

192 to 223

Could represent 254 hosts on one network

Class D address

224 to 239

Not relevant

Class E address

240 to 254

Not relevant

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