Add an IP address to a VPS and choose the primary
Add a second IPv4 address to a machine from the panel, make any address the primary, and have it set up inside the machine for you.
A customer who needs a second address asks for it on the machine's Network page. KLYRN gives it one from the right block and sets it up inside the machine as well.
Set up inside the machine
The new address is written into the machine's own network settings, so it answers straight away and still answers after a restart. Nobody has to sign in and edit a file.
Choose the primary
Any address on the machine can become its primary, the one it uses for outgoing traffic. The others stay as secondary addresses.
Within the plan
A customer can add as many addresses as their plan allows. A reseller spends their own allowance, and an administrator can add any address.
The docs cover address blocks and plans.