Understanding Routing

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Understanding Routing

Recall from the IP addressing and subnetting sections that the first thing a machine does when initiating communication with another machine is try to figure out whether the destination address is local or remote. It carries out calculations on the source and destination address based on the given subnet mask and then compares the two results. If the results are the same, the destination address is on the local network and ARP is then asked to get the physical address of the destination machine. If, however, the results do not match, the destination host is remote, and ARP is asked to get the physical address of a router or the default gateway.

Routers are devices that work at the Internet layer of the TCP/IP protocol suite, and have been designed to transfer or forward packets of data to their destinations, even when the routers themselves are not the destination. Consider for a moment that during a broadcast at the Network Interface layer (for example, ARP broadcast) for an IP address, presumably all the machines except one ignore the broadcast. This is because only one machine has that unique IP address. All other machines pass this data up to IP and then silently discard the data because the data is not intended for them. Routers are special machines that are told not to silently discard these kinds of packets, but to try to find the correct route or path to send messages when they receive messages not destined for them.

In this way, packets can traverse one network to another through a router, and because this routing process occurs at the Internet layer, it doesn’t matter what kind of network you’re running on, be it token ring, ethernet, or FDDI. Most machines are not designed to do this; only routers and gateways are. Figure 5.13 illustrates a simple routed network design with one router and three networks.

In this figure, the router separates the three network segments and keeps traffic on those network segments isolated from each other. Routers do not inherently support the passing of broadcasts, and must be specifically configured to do so on a network. This means that any machine broadcasting on network 1 will not be seen by machines on network 2 or 3. If a machine needs to communicate with a machine on the other side of a router, it needs to directly identify that machine and send those packets to the router to be forwarded.

How did this machine know the address of the router or default gateway? Host machines gain the IP address of the router in one of two ways. It is either manually configured in the network configuration of the machine, or the machine discovers the router address through a DHCP scope option. This information is stored in the registry and also appears in the machines internal route table.

This route table resides in memory and keeps track of networks and the physical interfaces that give access to those networks. On a local machine, the route table is fairly simple and usually contains no more than a few default entries, including the loopback address, the network on which the machine currently resides, and entries for various broadcasts. Figure 5.14 illustrates a common route table for a local machine.

Every machine on a TCP/IP network consults its route table to determine what to do with addresses that are destined either locally or remotely. If a destination is on the local network, the route table informs IP to send the data to the local machine. If, however, the destination address is remote, IP consults the route table for an entry specific to that network. If an entry specific to that network exists, the packet is sent to the network interface identified by that route table entry.

If no entry exists for that specific network and a default gateway has not been identified, the packet is dropped. It is only in the case that an entry does not exist and a default gateway for the machine has been configured, that IP will use the 0.0.0.0 route table entry (default gateway) to find a network interface address to send the data. Figure 5.15 has no specific networks identified and would therefore use the 0.0.0.0 entry, sending data destined for a remote network to 131.107.2.1 to be routed.

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