IP Renumbering - Avoid it if You Can
I'll admit that my experience is catered to web design and graphics and not so heavy in custom programming or server management. When it comes to custom server configuration or network management, I depend on the gurus around here like Mike or John.
Regardless, we all learned a lesson this week that we're going to have to pay tuition for in future months. The net lesson: have plenty of IP addresses allocated in advance before assigning to servers or client shared hosts.
Ok - let me explain. We host many customers. Some have their own dedicated server and some share a server with other customers. (side note: we don't host just anyone. Justgood quality clients - I'll share the reasons why on another post). Most "shared" clients are glad to also share the main server IP with other customers. In reality there's no reason for them not to. There are circumstances however when a customer needs their own dedicated IP. They may have an ecommerce site and in order to have their own SSL certificate for secure checkout, they need their own IP as well.
Dedicated server customer have to have their own IP to run their box.
So what's the problem? Well the organization ARIN regulates IP blocks. You can't just order a million IPs and hang on to them for future use. You have to order them in blocks of let's say 8, 16, 32, 64, etc. The regulation however stipulates that you can only have 2 active blocks at a time (unless of course you have a /24 block that has many usable IPs that are active). Why can you have multiple blocks if you have a /24 block? Well ARIN doesn't want IP's just thrown to and fro to little shops that may have interests that may be.... let's say illegal.
So they know the big players and cater to them. Since we only have smaller blocks, we can't just add a 3rd block. So in order to get more IPs when we run out, we have to swap out IPs that are in use with new IPs....they call it "renumbering IPs". In essence, you have to make sure your websites don't go down, while changing their IP's and updating the registrar if necessary. We're avoiding it for now because it's messy and has risk of downtime. When we do have to swap out blocks, we'll do it at night and on a weekend to limit risk during propogation.
So if you need blocks of IP's order the largest one you can get (and can afford) the first time. It'll save you hassle in the long run.