If you install XenServer on a dedicated server from www.ovh.ie it comes with two NICs defined. NIC1 is the “normal” bridge network that has the server public/management IP address. IF you want to add “normal” IPs that you’ve purchased then the normal OVH guides are very good.
However, it’s not particularly clear how to add an IP block to a vRack, I couldn’t find the documentation easily on the site. Their guides are good but can be a bit incomplete, with links to things in French etc.
Anyway, to cut a long story short: In the default XenServer install there’s a second NIC, NIC2, this NIC gets all the vRack traffic. So you can assign this NIC as the “eth0” for a VM and then add an IP from your block, with the default gateway as described in the e-mail.
It’s very easy, there’s none of the extra steps regarding changing default routes to go out through the NIC1 gateway, just treat the eth0/NIC2 as a regular interface.