TCP IP Illustrated

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Unformatted text preview: e this is a direct route (the G flag is not set) the gateway column specifies the IP address of the outgoing interface. file:///D|/Documents%20and%20Settings/bigini/Docum...i/homenet2run/tcpip/tcp-ip-illustrated/ip_rout.htm (4 of 17) [12/09/2001 14.46.50] Chapter 9. IP Routing Implied in this final entry, but not shown by the netstat output, is the mask associated with this destination address (140.252.13.32). If this destination is being compared against the IP address 140.252.13.33, the address is first logically ANDed with the mask associated with the destination (the subnet mask of the interface, 0xffffffe0, from Section 3.7) before the comparison. For a network route to a directly connected network, the routing table mask defaults to the subnet mask of the interface. But in general the routing table mask can assume any 32-bit value. A value other than the default can be specified as an option to the route command. The complexity of a host's routing table depends on the topology of the networks to which the host has access. 1. The simplest (but least interesting) case is a host that is not connected to any networks at all. The TCP/IP protocols can still be used on the host, but only to communicate with itself! The routing table in this case consists of a single entry for the loopback interface. 2. Next is a host connected to a single LAN, only able to access hosts on that LAN. The routing table consists of two entries: one for the loopback interface and one for the LAN (such as an Ethernet). 3. The next step occurs when other networks (such as the Internet) are reachable through a single router. This is normally handled with a default entry pointing to that router. 4. The final step is when other host-specific or network-specific routes are added. In our example the route to the host slip, through the router bsdi, is an example of this. Let's follow through the steps IP performs when using this routing table to route some example packets on the host svr4. 1. Assume the destination address is the host sun, 140.252.13.33. A search is first made for a matching host entry. The two host entries in the table (...
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