Basic Routing

Any system on a network can perform a router's basic functions if it is properly configured to route between networks, even if it has only a single network interface card (NIC) installed. This one NIC must have two IP addresses bound to it (both from different networks), and a route must be identified in the routing table between the two networks. If the system meets these two requirements, when it receives packets from one network destined for the other, it can successfully pass the packets along.

To see the local routing table for a local system, enter ROUTE PRINT at a command prompt; the output is shown in Figure 4.1.

Figure 4.1. The local routing table as it appears on most Windows Server 2003 systems with a single NIC.

The columns in the Active Routes section have different functions:

• Network Destination ” The addresses in this column are used as endpoints for network traffic.

• Netmask (subnet mask) ” This column is used to divide which part of the listed IP address is the network's numeric name and which part is the intended host's numeric name .

• Gateway ” This column denotes the default gateway address the host uses to send packets to other subnets.

• Interface ” This column lists the IP address for the network adapter that the host system uses to transmit data over the network.

• Metric ” This column assigns a cost to an interface and is usually considered when more than one network card is installed on a host; however, it can also be used to simply assign costs to different routes listed for a single NIC. A lower metric on a NIC or route denotes that it will be used preferentially. Other listings with higher metrics are used when the lowest cost route becomes unavailable.

Considerations for setting metrics include factors such as hop count, which is the number of routers crossed from the sending host to the destination system; each router crossed is another hop added. Other factors, such as throughput and reliability of LAN and WAN connections ( mainly considered based on their speed ratings) need to be considered as well.

When routes to other networks change because of temporary network failures or permanent changes in network paths, the corresponding routing entries on a router must be updated. Persistent routes on hosts entered into the local routing table must be deleted or at least updated. To add persistent routes to a host's local routing table, use the “p switch, as shown in the following lines in a command-prompt window:

 route  ADD  -p 10.0.0.0 MASK 255.0.0.0  10.55.80.1 METRIC 1 IF 2          destination^      ^mask       ^gateway   metric^    ^                                                     Interface^ 

If you need to change this persistent setting because of a temporary update to the indicated routing path , enter the following:

 route  CHANGE  -p 10.0.0.0 MASK 255.0.0.0  10.55.80.254 METRIC 1 IF 2             destination^      ^mask       ^gateway   metric^    ^                                                        Interface^ 

If the changes no longer require this host to hold a persistent route, delete the entry with the following command:

 route  DELETE  -p 10.0.0.0 MASK 255.0.0.0  10.55.80.1 METRIC 1 IF 2             destination^      ^mask       ^gateway   metric^    ^                                                        Interface^