On Mean Servers' network, IPv6 is statically assigned and pointed to a gateway that is outside the /64 allocated to you, meaning you must add a static route to point to the gateway. This isn't difficult, but different distributions of Linux have various ways of configuring the network. SolusVM is generally able to automatically setup networking during the initial setup, and for most configurations is able to reset networking from within the control panel, however this may not work for obscure configurations or if you don't want SolusVM to reset any manually applied settings.
NetworkManager Managed Systems
NetworkManager is increasingly becoming commonplace with many Linux distributions choosing to make it default. NetworkManager has many different interfaces to manage it, but they're all similar in look and functionality. From the command line, there is an ncurses-based interface called nmtui that makes it a snap to setup any connection.
To start, run nmtui as root
In the menu that shows up, use the arrow keys to move between menu selections, tab to move between window scops (i.e. menu to buttons such as <OK>), space to select or deselect check boxes, and enter to enter a menu or select a button. Go to Edit a connection and under Ethernet, Wired connection 1 should by default be selected. Click Enter to enter the edit screen. Scroll all the way down to IPv6 Configuration and open the menu that says Link-Local (it may also be Ignore or Automatic). Select Manual. To show the rest of the menu if it is hidden, select the button that says <Show>. In the addess field, enter the address that shows in SolusVM -> Manage -> Controls -> Network -> IPv6 Subnets -> Manage -> IP Addresses. For NetworkManager, this includes the subnet mask of /64. The Gateway is that listed in SolusVM near the top of the page. However, to access this gateway, we need a static route to that network. So go to Routing -> Edit -> Add in nmtui, and change the value to match the Gateway field, and add a /128 at the end. Add the DNS Servers listed in SolusVM. If you want IPv6 to act as the default for your OS to us, deselect Never use this network for default route. Hit <OK> then <Back>, and Quit. Issue systemctl restart NetworkManager, and your machine should now have IPv6.
Debian Based Systems (Debian/Ubuntu)
Most headless (without a GUI) installs of Debian-based distributions elect to stay with Debian's way of doing networking, which is to edit /etc/network/interfaces. Open that file in your editor of choice, and under your auto eth0, iface eth0 static stanza, add a new section with the following contents
iface eth0 static
address <enter your address here>
netmask 64
dns-nameservers 2001:4860:4860::8888 2001:4860:4860::8844
up ip -6 route add <enter your gateway here> dev eth0
up ip -6 route add defualt via <enter your gatway here>
The gateway and address can be found in SolusVM -> Manage -> Controls -> Network -> IPv6 Subnets -> Manage -> IP Addresses.
RHEL Based Systems (RedHat/CentOS/Scientific Linux)
For RHEL-based distributions networking is configured through /etc/sysconfig/network-scripts/ifcfg-eth0. To enable IPv6, add the following lines
IPV6INIT="yes"
IPV6ADDR="<enter your address here without the /64>"
DNS1="2001:4860:4860::8888"
DNS2="2001:4860:4860::8844"
We need to add the route in a seperate file, /etc/sysconfig/network-scripts/route-eth0. Add the following lines:
route add <enter your gateway here> dev eth0
route add default via <enter your gateway here> dev eth0
The gateway and address can be found in SolusVM -> Manage -> Controls -> Network -> IPv6 Subnets -> Manage -> IP Addresses.