tailieunhanh - Internetworking with TCP/IP- P12

Internetworking with TCP/IP- P12: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | 78 Mapping Internet Addresses To Physical Addresses ARP Chap. 5 machine B across a physical network to which they both attach but A has only B s internet address Ib. The question arises how does A map that address to B s physical address Pa Address mapping must be performed at each step along a path from the original source to the ultimate destination. In particular two cases arise. First at the last step of delivering a packet the packet must be sent across one physical network to its final destination. The computer sending the packet must map the final destination s Internet address to the destination s physical address. Second at any point along the path from the source to the destination other than the final step the packet must be sent to an intermediate router. Thus the sender must map the intermediate router s Internet address to a physical address. The problem of mapping high-level addresses to physical addresses is known as the address resolution problem and has been solved in several ways. Some protocol suites keep tables in each machine that contain pairs of high-level and physical addresses. Others solve the problem by encoding hardware addresses in high-level addresses. Using either approach exclusively makes high-level addressing awkward at best. This chapter discusses two techniques for address resolution used by TCP IP protocols and shows when each is appropriate. Two Types Of Physical Addresses There are two basic types of physical addresses exemplified by the Ethernet which has large fixed physical addresses and proNET which has small easily configured physical addresses. Address resolution is difficult for Ethernet-like networks but easy for networks like proNET. We will consider the easy case first. Resolution Through Direct Mapping Consider a proNET token ring network. Recall from Chapter 2 that proNET uses small integers for physical addresses and allows the user to choose a hardware address when installing an interface board in a .