Virtual IPs/Multiple IPs on one Network Interface (Debian/Ubuntu)
To add additional IPs to a single network interface add the example config below to your /etc/network/interfaces file:
auto eth0:0 iface eth0:0 inet static address 192.168.40.200 netmask 255.255.255.0 network 192.168.40.0 broadcast 192.168.40.255 gateway 192.168.40.1 auto eth0:1 iface eth0:1 inet static address 192.168.50.200 netmask 255.255.255.0 network 192.168.50.0 broadcast 192.168.50.255 gateway 192.168.50.1
This presumes that your existing interface is called eth0. It can be checked by running the ifconfig command. Ensure that your virtual interfaces start with a 0 and not a 1.