A feature of an Internet account
, where the user is guaranteed to be assigned the same IP address
each time they log in
to their ISP.
In the 'good old days', when the internet wasn't so crowded (I'm talking of around 1994 and before, at least in the UK, probably a year or two before that in the States) each dialup customer using an ISP generally had their own IP address, reserved for their use. This was tremendously wasteful, but the IP address supply hadn't then reached today's crisis point, and ISPs had large network blocks assigned to them. And it was a heck of a lot easier for the user to just be told 'their' IP address and type it into their tcp/ip connector (in all likelihood, this was Trumpet Winsock for Windows 3.x, or, if they were still running DOS, something like k9q) once and for all than it was to set up what's called 'dynamic PPP'.
As the IP address shortage, unimagined by the initial designers of IP, and fuelled by the sudden mainstream appeal of the Internet, began to bite, the Authorities, such as RIPE (in Europe) and ARIN (in the States) began to hold back on new assignments of netblocks (groups of IP addresses) to the rapidly growing ISPs attempting to cater to the sudden demand, and to insist that the owners of the new netblocks met certain conditions about their use.
One of the first conditions imposed was that the practice of wholesale distribution of 'static IP addressing' (as the old method came to be called) had to end. The basis for this is that, at any one time, only about 1/10 to 1/30 of the ISP's customers (or less, during slack hours) would be online, thus the ISP was using about 20 times as many IP addresses as they needed to.
So the practice of 'dynamic IP address allocation' was introduced, and authentication servers (such as RADIUS) or access concentrators (big 'multi-modem' servers like the Ascend MAX 4000 that you plug multichannel telephone cables into) were configured to dish out an address from a 'pool' belonging to the ISP, instead of a specific address configured in the cutsomer's account.
ISP customers being what they were, this did not go down well with the vocal minority. Whereas previously a customer had been consistently getting the same IP address, and may have had a vanity hostname assigned to it in DNS. which would identify them on IRC and allow them to run DNS-lookupable servers on their home machines, now they would get a different IP address each time they logged in, and probably this would resolve to some anonymous-sounding hostname like ppp-192-168-45-32.isp.com. Furthermore, some people who were working from home, perhaps, or for other reasons needed access to a secured server somewhere, where authentication by IP address was part of the security, found this would no longer work.
And so ISPs would arrange (sometimes at a premium, sometimes just if the user was intelligent enough to work out who to ask) for some dialup accounts to be allocated a static IP address, as opposed to the new standard dynamic IP addresses. This, for some people, became a prized (or sometimes simply essential) addition to their ISP account, for any number of reasons, including the ones outlined above. Network numbering authorities like RIPE recognised the validity of the need, and would accept that a certain proportion of addresses assigned could be used for this purpose (though it was (and still is) required to state the proportion in the applications made.)
Today, as we inch towards universal broadband access, with 24 hour connectivity becoming the standard (though, in the States, where local phonecalls are often free, this has been the long practice of a select few individuals since the early days of the commercial Internet) the feature is still valued by users who want to run their own webservers and mailservers at home, since they can buy DNS services (perhaps from a third party) which resolve their chosen hostname to the static IP. Though dynamic DNS (which allows the DNS to update itself each time they are assigned a different dynamic addresss when they log on) is a theoretical possibility, and has actually been put into practice in a few places, it is a clumsy and inefficient solution for people wishing to run highly available servers from home, and for the online workforce, in today's paranoid network environment, VPNs (virtual private networks) are often simpler to set up when static addresses are used.
Perhaps when we all migrate to IPng (or IPv6) with its hugely expanded address space, we will be in a new 'good old days' and everyone will have a static address again. But until then, the static IP address will likely remain a desirable feature of an ISP account for a sizeable proportion of the technically literate end of the market.