right © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25, Internets, Intranets, and Extranets X.25, Internets, Intranets, and Extranets Class B address, before subnetting Figure 7-23 Subnet address for Class B class B. Network Class B address, after subnetting Class B Network Subnet Class A Figure 7-24 addresses A subnet mask for class A address. 00100010 34.0.0.0 Subnet mask 8 subnet bits 255.255.0. 0 ple, address 192.168.1.0 refers to network 192.168, subnet 1; address 192.168.2.0 refers to network 192.168, subnet 2; and SO on. The number of bits borrowed for the subnet address varies. To specify how many bits are used, IP provides the subnet mask. Subnet masks use the same format and representation technique as IP addresses. Subnet masks have ones in all bits except those bits that specify the host field. For example, the subnet mask that specifies 8 bits of subnetting for class A address 34.0.0.0 is 255.255.0.0, as shown in Figure 7-24. Internet Routing Routing devices in the Internet have traditionally been called gateways- an unfortunate term because elsewhere in the industry the term applies to a device with somewhat different functionality. Gateways (which we will Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25, Internets, Intranets, and Extranets Chapter 7 call routers from this point on) within the Internet are organized hierar- chically. Some routers are used to move information through one particular group of networks under the same administrative authority and control (such an entity is called an autonomous system). Routers used for informa- tion exchange within autonomous systems are called interior routers, and they use a variety of Interior Gateway Protocols (IGPs) to accomplish this purpose. Routers that move information between autonomous systems 