IP Aliasing is the process of adding additional IP addresses to a single network interface.

Linux[edit]

Use the ip command to add or remove IP addresses from an interface.

# ip address add $ip/$mask dev $interface
# ip address del $ip/$mask dev $interface

To make it persistent, create a ifcfg script with the IP changes:

# cp /etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network-scripts/ifcfg-eth0:0
# cat /etc/sysconfig/network-scripts/ifcfg-eth0:0
DEVICE=eth0:0
NAME=eth0:0
IPADDR=10.1.1.2
NETMASK=255.255.255.0
ONBOOT=yes

Ensure that there is no GATEWAY set in the ifcfg-eth0 file because it should be set in /etc/sysconfig/network.

FreeBSD[edit]

Under FreeBSD, to add an additional IP address (10.1.1.2, with a netmask of 255.255.255.0) to re0, run:

# ifconfig re0 10.1.1.2 netmask 255.255.255.255 alias

The changes above are not persistent and will not survive a reboot. Add the following to /etc/rc.conf to make it persistent:

# ifconfig_re0_alias0="10.1.1.2 netmask 255.255.255.255 alias"