Indirect routing example

The following procedure shows how packets are routed to a system not connected directly to the local network. See Figure 5-1, ``Example internetwork'', for an illustration of the networks described in this example.

1. The IP layer of the machine thames receives a packet addressed to the machine paris at the IP address 132.147.246.3

2. thames consults its kernel routing table, which may look like the following:

      destination     netmask          gateway         interface
      132.147.246.0   255.255.255.0    volga           le0
      132.147.118.0   255.255.255.0    local           le0
      default         N/A              volga           le0
   

3. As described in the previous example, thames applies each netmask to the IP address 132.147.246.3 until it finds a match with the destination address. Having found a destination match, thames uses the gateway and interface fields of the entry. thames addresses the packet for the gateway. In this case, the gateway is the machine volga.

4. thames transmits the packet through the specified interface. In this case, the interface is to an Ethernet, so thames does a lookup in the ARP table to translate the IP address for volga to an Ethernet address for volga. thames transmits the packet on the Ethernet, and it is received by volga.

5. When volga receives the packet, it reads the ultimate destination IP address. Finding that the address is not its own, and because volga is configured as a router, it consults its kernel routing table as thames did above. volga finds that the ultimate destination address can be reached via the local gateway and sends the packet out the local Ethernet interface addressed to the ultimate destination IP address.

6. paris receives the packet.

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