makarmi In the command line with this command
ifconfig
you can see network cards and its IP address, each device has a name, if you have only one network card you vwill see eth0, if you have two network cards you will see eth0, eth1, and so on.
I am gonna try to explane myself as clear as i can, and sorry for my English
If you want to configure the IP address for the eth0 network card you have to write this command
#ifconfig eth0 192.168.5.2 netmask 255.255.255.0
If you want to configure the IP address for the eth1 network card you have to write this command
#ifconfig eth0 192.168.6.2 netmask 255.255.255.0
But if you only have one network card and you want this card have more than one IP address you have to write this command, this is commonly called IP ALIAS, subneting is another subject that i do not talk about in this answer
#ifconfig eth0 192.168.5.2 netmask 255.255.255.0
#ifconfig eth0:0 192.168.6.2 netmask 255.255.255.0
If you notice I did not use any gateway, but in the fact you have two different IP addresses you probably need to change between the two gateways and you can do this with this commands
#route add default gw 192.168.5.254
In this case we asume your default gateway is the gateway/router in the first network, and if you want change your gateway for the second network address, you have to delete first your current gateway with this command
#route del default
And after adding the new one
route add default gw 192.168.6.1
