Adding an IP address
If you've been assigned one or more additional IP addresses, here is how to add them.
You can use the ip command to make an additional IP address work immediately:
# ip address add 220.127.116.11/32 dev eth0
This address should now respond to ping and be completely usable (unless you have firewalling preventing that).
This configuration won't last past the next reboot however, so you now need to configure the change permanently.
Debian / Ubuntu
First open your interfaces file for editing:
# nano /etc/network/interfaces
You should see your main IP address described for IPv4 and IPv6. The IPv4 part looks something like:
# The primary network interface auto eth0 iface eth0 inet static address 18.104.22.168 netmask 255.255.248.0 gateway 22.214.171.124
Go to the end of the file and add information about your new IP. It doesn't need a gateway and you can use 255.255.255.255 for netmask:
# The secondary network interface auto eth0:0 iface eth0:0 inet static address 126.96.36.199 netmask 255.255.255.255
For each additional IP, use eth0:1, eth0:2 etc.
Once you're finished, press Ctrl-X and save your file.
Shutdown and boot
If you want to test your permanent configuration then you'll need to shut down and boot your VPS again to ensure it comes up properly.
Due to a curiosity of the way Xen creates virtual machines, the next time you want to restart your VPS you should do a shut down and then a boot, rather than a reboot. If you reboot then Xen will only route IP addresses to you that you had last time it was booted, i.e. not any that you've had added since. If you forget and reboot anyway then you can restore the additional IP address's route by shutting down and booting again as described.
# shutdown now
$ ssh email@example.com
Once logged in you will see the Xen command line prompt. Try
Xen shell> boot
If it complains that an instance already exists, and you've waited sufficiently long that the machine should have shut down, do
Xen shell> destroy
Xen shell> boot
You can now log back in to your VPS and your new IPs should work.