Data Communications Networking Devices - 4th Ed.pdf

218 __________________________________________________________ WIDE AREA NETWORKSnames are much easier to remember and use, routing is based upon IP addressingwith the DNS providing the mechanism required to translate DNS names into IPaddresses. Since IP addresses form the basis for network addressing as well as theuse of DNS, we will ®rst focus our attention upon that topic prior to examining therole of the DNS.IP addressingThe IP addressing scheme uses a 32-bit address which is divided into an assignednetwork number and host number. The latter can be further segmented into asubnet number and host number. Through subnetting you can construct multiplenetworks while localizing the traf®c of hosts to speci®c subnets, a technique I willshortly illustrate.IP addressing numbers are assigned by the InterNIC network informationcenter and fall into one of ®ve unique network classes, known as Class A throughClass E. Figure 2.29 illustrates the IP address formats for Class A, B and C networks.Class D addresses are reserved for multicast groups, while Class Eaddresses are reserved for further use.In examining Figure 2.29, note that the ®rst bit in the IP address distinguishes aClass A address from Class B and C addresses. Thereafter, examining the compositionof the second bit position enables a Class B address to be distinguishedfrom a Class C address.An IP 32-bit address is expressed as four decimal numbers, with each numberranging in value from 0 to 255 and separated from another number by a dotdecimal point). This explains why an IP address is commonly referred to as adotted decimal address.Figure 2.29IP address formats