How do I add an additional IP address to /etc/network/interfaces?

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
  address aaa.aaa.aaa.aaa
  netmask 255.255.254.0
  gateway bbb.bbb.bbb.bbb
  dns-nameservers ccc.ccc.ccc.ccc ddd.ddd.ddd.ddd eee.eee.eee.eee
  dns-search vps-number.com

auto eth0:0
iface eth0:0 inet static
  address fff.fff.fff.fff
  netmask 255.255.254.0

Then you can run sudo ifup eth0:0 to bring it up and sudo ifdown eth0:0 to bring it down.

You can just have repeated iface stanzas for the same interface. Example from https://wiki.debian.org/NetworkConfiguration#Multiple_IP_addresses_on_one_ :

auto eth0
allow-hotplug eth0
iface eth0 inet static
    address 192.168.1.42/24
    gateway 192.168.1.1

iface eth0 inet static
    address 192.168.1.43/24

iface eth0 inet static
    address 192.168.1.44/24

# adding IP addresses from different subnets is also possible
iface eth0 inet static
    address 10.10.10.14/24

So just the solution above but drop the :x suffix, which as Heihachi points out is outdated.

(The ip addr suggestion is the worst. It's ugly and incomplete, as you'll have to also add a down variant or ifdown won't work very cleanly.)

For an extra IP address, I usually add:

up ip addr add fff.fff.fff.fff/prefixlen dev $IFACE

to the bottom of the iface eth0 inet static stanza for future reboots and then run the command sudo ip addr add fff.fff.fff.fff/prefixlen dev eth0 again manually to activate it directly.

If your netmask is 255.255.254.0 then prefixlen should be 23 for you.

I'd love to know if there's a better way, though.