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Why Do ISPs Change Your IP Address?


If you rely on any sort of service that requires you to know the IP address of your home internet connection, there’s a good chance you’ve noticed that the number (however frequently or infrequently) changes. Why is that?

Image courtesy of EasyDNS, a dynamic DNS service provider.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader Agz is curious why his ISP just doesn’t give him a fixed IP address:

Is there any specific reason an ISP will need to change your IP address? What is the purpose of a dynamic IP versus a static IP? For me it seems to happen every 6 months, while for someone I know, it does once a week.

Why indeed? Why not just assign a permanent address to every customer?

The Answer

SuperUser contributor Flimzy offers some insight into the IP assignment methods:

When ISPs were first starting, everyone connected to the Internet over a modem. And most people used the Internet for a few minutes to a few hours per week. Assigning a static IP to every subscriber would have been very expensive, for something that most people used just a few minutes a week.

As broadband connections have become more common, the practical reasons for not assigning a static IP have become much less noticeable, as now the majority of connections are “always-on”–even when nobody is (actively) using the Internet.

So there’s a bit of a historical reason not to use static IPs–customers are already accustomed to using dynamic IPs.

When modern ISPs enforce dynamic IPs these days, it may be in part to distinguish between “consumer” and “professional” services–by reserving static IPs for customers who pay more, it gives customers who need that feature an incentive to upgrade their service level.

It can also serve as a deterrent for people abusing their consumer-grade service. Many ISPs, for instance, explicitly prohibit running “servers” on a home Internet connection. If every home user had a static IP, they’d be more inclined to abuse such terms of service.

It’s also less of a management problem to assign customers dynamic IPs. If you move across town (but within the same ISP’s service area), there’s no need to re-assign how your static IP is routed; you’ll just get a dynamic IP that exists in the new neighborhood.

Now that you know why you routinely receive a different IP address, check out our article How To Easily Access Your Home Network From Anywhere With DDNS to configure your home network to use a free dynamic DNS service to easily find your way home no matter how many times your ISP changes your IP.


Have something to add to the explanation? Sound off in the the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

 

Jason Fitzpatrick is warranty-voiding DIYer and all around geek. When he's not documenting mods and hacks he's doing his best to make sure a generation of college students graduate knowing they should put their pants on one leg at a time and go on to greatness, just like Bruce Dickinson. You can follow him on if you'd like.

  • Published 06/4/13

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