Direct routing example

The following procedure shows how packets are routed to a system 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 seine at the IP address 132.147.118.4.

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. thames applies each netmask to the destination IP address 132.147.118.4 until it finds a match with the destination address.
   That is, the four places of the IP address are aligned over the four places of the netmask and the IP address is pushed through the netmask as through a filter. The number strings 255 allow the IP address number to pass through unchanged. The zeros in the mask convert the IP address string to zero as shown below.
   
   132.147.118.4
   255.255.255.0
   results in
   132.147.118.0

   As you can see, the result matches the second destination in the routing table. (If no match is found, thames uses the default entry.)

4. 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 local, meaning the local network, so the ultimate destination address is used. 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 seine to an Ethernet address for seine. thames transmits the packet on the Ethernet, and it is received by seine.

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