tailieunhanh - The Illustrated Network- P29

The Illustrated Network- P29:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 9 Forwarding IP Packets 249 Management Management Terminal Terminal FIGURE The three router access methods. Note that the console port requires access to the router while the others allow remote access. called VTY lines on a Cisco router with a more secure remote access program called secure shell SSH using a Web browser HTTP is the protocol or with SNMP Simple Network Management Protocol a protocol invented expressly for remote router management. These arrangements are shown in Figure . Small routers usually only have a console port. With the proper cables these console ports can be hooked up to a modem for remote access but obviously cannot be used simultaneously for local access. On some routers the console ports are labeled Admin or Management. It is tempting to try and access a console or AUX ports using the normal graphical interface provided by Windows a Mac or Unix X-Windows. But the console and AUX ports only understand a simple character-based serial protocol. On Windows PCs for example only HyperTerminal or another serial terminal emulation program can communicate with a router through the console or AUX ports. FORWARDING TABLE LOOKUPS In the connectionless best-effort world of IP every packet is forwarded independently hop by hop toward the destination. Each router determines the next hop for the destination address in the packet header based on information gathered into the routing table and distilled into the forwarding table. The essential operation of a router is the looking up of the packet s destination IP address in this table to determine the next hop. 250 PART II Core Protocols It s unusual that a packet address is an exact match for a table entry. Otherwise routing and forwarding tables would need an entry for every host in the world all 32 bits for IPv4 and 128 bits for IPv6 So in the current classless prefix world of IP addressing the host-hop destination is chosen by the longest match rule. Figure shows how the next-hop .