In order to understand routing on our host we are going to need to use the netstat command.

netstat can do all sorts of things, but probably the most useful thing is the -r option and that will show us the routing tables.

If you run the netstat -r command, after you have plumbed your interface, you should see at least the following:

linux:~ # ifconfig eth0:1 172.16.5.112 netmask 255.255.255.0
linux:~ # netstat -r
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
172.16.5.0      *               255.255.255.0   U         0 0          0 eth0
192.168.1.0     *               255.255.255.0   U         0 0          0 eth0
default         192.168.1.1     0.0.0.0         UG        0 0          0 eth0
linux:~ #
            

You should see a table and in your destination you should see 192.168.0.0 which would be the network. The gateway would be 0.0.0.0 the mask would be whatever the mask was set to, in this case a Class C mask, the flag will indicate a "U" to show that the network is up and the iface should indicate eth0

You should see an entry for the 172.16.5 network and finally if you've got a default gateway set you should see an entry called 0.0.0.0 (or whatever your gateway was set to).

The difference in this line is that your destination will be 0.0.0.0 which indicates the default gateway.

Remember that the default gateway is the place we send network traffic in the event that we don't know where else to send it.

The default gateway is currently up (indicated by the 'U' flag) but the flag also indicates a 'G' which shows that it is a gateway and the interface is eth0. Our routing table tells us where and how to route information around the network.

Now we know how to look at our routing table, we will also need to know how to add routes to it. Most importantly, we will need to add a default route to our host. Again, like configuring the IP address on-the-fly we can configure the default gateway on-the-fly too using the route command:

route add default gw 192.168.1.1 netmask 255.255.255.0
            

Consult your routing table again to verify that this is indeed the default gateway now. Not only can we add the default gateway, we can also add networks that we may know about. Assume for a minute that there is another network 10.121.20.x to which we can gain access, but not directly. In other words, we have a host through which we route in order to gain access to this network. Assume too that this hosts is on our local network, with the IP address 192.168.1.100. Now we can add a route to our network to indicate to our frames (or packets) how to get to hosts on this network (10.121.20.x). We could do this as follows:

route add -net 10.121.16.0 netmask 255.255.248.0 gw 172.16.1.1 metric 1
            

Note here we need to provide the route with the correct network address for the netmask that we are supplying [head back to your ipcalc program to verify this]. The gw (gateway) indicates the host that will accept frames on behalf of this network and the metric indicates how many hops we will need to do prior to getting on this network.

So much for on-the-fly configuration. Setting up your networks and especially your default gateway on your hosts permanently, you need to edit a file. On RedHat Linux (and Fedora) you will need to edit the /etc/sysconfig/network-scripts/ifcfg-eth0 and add the word GATEWAY=<your default gateway>. On Debian, this is configured in the interfaces file in /etc/ and on SuSE it is configured using YaST.