Yes you can use the whole /64 of IPv6 on OVH's Kimsufi
Just a quick heads up to those who bought one the latest Kimsufi servers.
These servers are advertised to come with a /128 of IPv6 (one address).
However this address follows a specific format:
If you are familiar with IPv6, this looks exactly like the first IP in a /64 (all remaining bits of the "host" part set to zero). Looks like everyone has their /128 IPv6 ending with ::1 (if that's not the case for someone, please post). So they seem to allocate a /64 per customer, then only configure one (first) IP out of it on the server by default.
Let's try adding a second IP to the interface:
ip addr add 2001:41d0:x:abcd::2/128 dev eth0
Yep, it works totally fine, pingable from the outside and everything.
So in case you just need 2-3 more IPv6 addresses, this is not a problem whatsoever, just add them in a similar form, they will work, and you don't risk "colliding" with another customer. However there's no way to define rDNS records for those additional IPs, so their usefulness remains somewhat limited.