Data Communications Networking Devices - 4th Ed.pdf

2.5 THE INTERNET _______________________________________________________________ 219Class AIn examining Figure 2.29, note that a Class A address has three bytes available foridentifying hosts on one network or on subnets which provides support for morehosts than other address classes. Thus, Class A addresses are only assigned to largeorganizations or countries. Since the ®rst bit in a Class A address must be zero, the®rst byte ranges in value from 1 to 127 instead of to 255. Through the use of 7 bitsfor the network portion and 24 bits for the host portion of the address, 128networks can be de®ned with approximately 16.78 million hosts capable of beingaddressed on each Class A network.Class BA Class B address uses two bytes for the network identi®er and two for the host orsubnet identi®er. This permits up to 65 536 2 16 ) hosts and/or subnets to beassigned; however, since the ®rst two bits of the network portion of the address areused to identify a Class B address, the network portion is reduced to a width of 14bits. Thus, up to 16 384 2 14 ) class B networks can be assigned. Due to the mannerby which Class B network addresses are subdivided into network and host portions,such addresses are normally assigned to relatively large organization withtens of thousands of employees.Class CIn a Class C address three octets are used to identify the network, leaving one octetto identify hosts and/or subnets. The use of 21 bits for a network address enablesapproximately 2 million distinct networks to be supported by the Class C addressclass. Since one octet permits only 256 hosts or subnets to be identi®ed, manysmall organizations with a requirement to provide more than 256 hosts with accessto the Internet must obtain multiple Class C addresses.Host restrictionsIn actuality the host portion of an IP address has two restrictions which reducesthe number of hosts that can be assigned to a network. First, the host portioncannot be set to all zero bits as an all-zeros host number is used to identify a basenetwork or subnetwork number. Secondly, an all-ones host number represents thebroadcast address for a network or subnetwork. Thus, the maximum number ofhosts on a network must be reduced by two. For a Class C network a maximum of254 hosts can then be con®gured for operation.SubnettingThrough the use of subnetting you can use a single IP address as a mechanism forconnecting multiple physical networks. To accomplish subnetting you logicallydivide the host portion of an IP address into a network address and a host address.