Online.net’s french tutorial is here: http://documentation.online.net/fr/serveur-dedie/reseau/ip-failover
Be careful if you use Google Translate on the page. The translation gets done on the codes too, and breaks them.

First, visit your control panel on online.net: https://console.online.net/
On Manage the Server option, click on “Add IP address”. Order an IP. Note that you will be charged for it at the next bill.
Edit IP Failover.
There are two boxes here. On the left is the list of failover IPs you’ve purchased, and on the right is the label of your server. Drag your failover IP from the box on the left to the one on the right.

Now configure your server, and add the following to /etc/network/interfaces:

#Failover configuration
auto dummy0
iface dummy0 inet static
  address 6.6.6.6
  netmask 255.255.255.255

Replace 6.6.6.6 by your IP.
Changes take place within 5 minutes of editing the options in the control panel.