tailieunhanh - The Illustrated Network- P27

The Illustrated Network- P27: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 8 Routing 229 Table Frame IP and MAC Addresses MAC Source Address IP Source Address MAC Destination Address IP Destination Address Frame 00 b0 d0 45 34 64 00 05 85 8b bc db leaving Linux client Linux client Juniper router Linux server client Frame arriving at server 00 05 85 88 cc db Juniper router Linux client 00 d0 b7 1f fe e6 Linux server Linux server Now if the MAC address associated with the frame leaving the client is 00 bo do 45 34 64 then the MAC address associated with the same IP address on the server LAN cannot magically change to 00 05 85 88 cc db. As expected the IP packet is identical except for the decremented TTL field but the frame is different. This is sometimes called indirect delivery of packets because the packet is sent through one or more network nodes and not directly to the destination. These relationships are displayed in Table which shows how the MAC addresses relate to the IP subnet addresses. Tethereal not only gives the MAC addresses but also parses the 24-bit OUI and helpfully lists Intel as the owner of 00 d0 b7 and Juniper as the owner of 00 05 85. We can verify this on the Linux client or server. Let s look at the client s ARP cache. root@lnxclient admin sbin arp -a at 00 05 85 8b bc db ether on eth0 root@lnxclient admin The question mark just means that our routers do not have names in DNS. The Illustrated Network uses two small LAN switches for LAN1 and LAN2 but the nodes used for internetworking are routers. Let s take a closer look at just what a router does and how it delivers packets from LAN to LAN over an internetwork. Routing Routing is done entirely with IP addresses of course. Many books make extensive use of the concepts of direct routing and indirect routing of packets. This can be confusing since direct routing of packets does not require a router. In this chapter the terms direct delivery and indirect delivery are used instead.