Multiple IP addresses on One Interface

Interface aliasing allows one interface to have multiple IP addresses. This is useful when more than one server is to be visible via the Internet. Note that virtual hosts can support multiple Apache servers with a single IP address. Apache responds to the domain name supplied by the client in the HTTP header. In many other situations, one external IP is needed for each server using a port.

This /etc/network/interfaces text assigns three IP addresses to eth0. 

auto eth0
allow-hotplug eth0
iface eth0 inet static
    address 192.168.1.42
    netmask 255.255.255.0
    gateway 192.168.1.1

auto eth0:0
allow-hotplug eth0:0
iface eth0:0 inet static
    address 192.168.1.43
    netmask 255.255.255.0

auto eth0:1
allow-hotplug eth0:1
iface eth0:1 inet static
    address 192.168.1.44
    netmask 255.255.255.0

An alias interface should not have "gateway" or "dns-nameservers"; dynamic IP assignment is permissible.

The above configuration is the previous traditional method that reflects the traditional use of ifconfig to configure network devices. ifconfig has introduced the concept of aliased or virtual interfaces. Those types of virtual interfaces have names of the form interface:integer and ifconfig treats them very similarly to real interfaces.

Nowadays ifupdown uses ip utility from the iproute2 package instead of ifconfig. The newer ip utility does not use the same concept of aliases or virtual interfaces. However, it supports assigning arbitrary names to the interfaces (they're called labels). ifupdown uses this feature to support aliased interfaces while using ip.

Also, ifupdown supports specifying multiple interfaces by repeating iface sections with the same interface name. The key difference from the method described above is that all such sections are treated by ifupdown as just one interface, so user can't add or remove them individually. However, up/down commands, as well as scripts, are called for every section as it used to be.

This /etc/network/interfaces text assigns three IP addresses to eth0.

auto eth0
allow-hotplug eth0
iface eth0 inet static
    address 192.168.1.42
    netmask 255.255.255.0
    gateway 192.168.1.1

iface eth0 inet static
    address 192.168.1.43
    netmask 255.255.255.0

iface eth0 inet static
    address 192.168.1.44
    netmask 255.255.255.0

# adding IP addresses from different subnets is also possible
iface eth0 inet static
    address 10.10.10.14
    netmask 255.255.255.0

Additional information can be found on the http://www.shorewall.net/Shorewall_and_Aliased_Interfaces.html page.