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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.

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Because they have more customers than they have IPv4 addresses. –  Martin Schröder May 2 '13 at 18:56
    
Because maintaining lists of static addresses and who they are assigned to is a Royal PITA. Especially when the customer base isn't static. It's a lot easier to put them in a pool and let a server lease them out. –  Fiasco Labs May 7 '13 at 16:22

8 Answers 8

up vote 42 down vote accepted

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.

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I never thought of that - "historical reasons... people used to dynamic" - it's a nice one! +1 –  itsols May 2 '13 at 12:57
    
And it isn't strictly true. My dial-up connection always gave me the same IP address. –  Bill Michell May 2 '13 at 16:38
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@BillMichell: Static dial-up accounts certainly existed. They were rare--and especially rare if you didn't pay for them! –  Flimzy May 2 '13 at 20:51
    
Aside from providing an incentive to customers to upgrade and for making sure that home servers are not being run (I am running one :P), what would make it "very expensive" to give each subscriber a static ip? –  agz May 3 '13 at 2:45
    
If every home user had a static IP, they'd be more inclined to abuse such terms of service. - Would it really? I'd think that most people running a server would know to get a free dynamic address. I certainly saw a lot of such addresses back in the centralized P2P times (e.g. eMule, DC++). –  André Paramés May 3 '13 at 8:17

I can only speak from the point of a system administrator with a small to medium sized network. But this is a complicated topic so I am going to try to answer in the simplest fashion.

The goal of using dynamically assigned IPs is that most clients do not need the benefits of having a static IP. In your question, connections that would need a static IP would be customers that are hosting services like web servers, email, remote desktop, VPN, etc. The reason for this is that in order for machines to find them on the Internet, they would need to be able to track back to that IP by looking up the IP via DNS. For most home customers, there is no need for this so there is no reason to assign a home user a static IP.

The IP lease time is set by the ISP and typically if your device is still online and requests a new lease, the ISP will give you back the same IP. As nothing is gained by giving you a different IP. This also lets the ISP move blocks of IPs around and do other maintenance without interupting services. Most routers/modems have a display for the WAN IP and typically will display the lease time. It is usually a few hours to a few days. Odds are, your modem asks for an IP regularly, but the DHCP server simply hands it back the same address. When it has a reason NOT to, your IP will be updated with whatever new IP is being handed out.

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And because customers come & go. Since IPv4 leases are in short supply they need to be recovered quickly if a client stops using the ISP's services. It'll be reassigned to a new customer automatically. –  Matt H May 2 '13 at 3:20
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It also allows ISPs to provide services at different price points. A Static-IP is most often an extra charge for the customer. –  Andrew Lewis May 2 '13 at 18:00
    
And some ISPs will, thankfully, simply lock in your current Dynamic IP range on request so you don't have to set up an "actual" static ip. –  Dalin Seivewright May 2 '13 at 19:14

Static IP addresses are much more difficult to manage. Even most "static" IP addresses are assigned dynamically through DHCP using static reservations, but with either a statically configured IP address or a DHCP static reservation, if the client connection device changes (hardware replacement, upgrade, etc), then someone needs to make a change either on that device or in the DHCP configuration. Dynamic assignment avoids this problem.

Using DHCP also simplifies the network design and allows for easier changes later. If the ISP chooses to make changes to the way they are using their IP addresses, they can do so without making manual changes on client connection devices. Examples of such changes could include any of the following (or others):

  • Increasing the size of the network (moving from a /24 network to a /23)
  • Decreasing the size of the network (moving from a /24 to a /25)
  • Changing the network in use entirely (moving from 10.0.0.0/24 to 192.168.0.0/24)
  • Changing the gateway IP address
  • Changing the DNS server addresses

DHCP also allows for the ISP to provide additional configuration to some devices. For instance, if they want to provide images/configuration on a TFTP server, this can be provided to the device through DHCP.

Ultimately, it is much easier to manage and provides less complications to administering the network.

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"most "static" IP addresses are assigned dynamically through DHCP" Citation needed, IMO. Anecdote: When I got a static IP for my home ADSL line in 2005-ish, the ISP (fairly big consumer broadband ISP) included a sheet of paper with the IP address, netmask, DNS and gateway settings, which I had to enter manually. No DHCP at all. I'm pretty sure I kept that IP address until I cancelled the service at that location in 2011. –  Michael Kjörling May 2 '13 at 14:26
    
With business class service (often with IT staff), your case would be more common. In the majority of consumer cases, one of two cases is generally true. First, the ISP controls the gateway device, in which case they can set up the static reservation quite easily. Second, it is easier to configure the static reservation than to explain to a user how to configure the static IP and support issues that arise from the manual method. Certainly there are ISPs that do it both ways, but static reservations cost less (in time/effort) and allow for flexibility that the manual configuration does not. –  YLearn May 2 '13 at 16:19
    
If a customer pays (I dare say that when it's offered, it usually comes with a specific fee) for a static IP, the ISP can't change it without giving the user plenty of prior notice, because the user may very well depend on it being static elsewhere. –  Michael Kjörling May 2 '13 at 20:41
    
You are absolutely correct in that statement, however they can change the size of the network the IP is part of (i.e. netmask), gateway, or assigned DNS servers without any impact on the end users use of the actual IP address. Other information could be provided to the gateway device through DHCP as well, if it is owned by the ISP. –  YLearn May 3 '13 at 0:44
    
@MichaelKjörling That sounds a lot like Speakeasy. Popular among geeks, but not exactly a "big consumer ISP". Making non-techies enter all that configuration data by hand is a formula for generating a ton of support calls, and that's a cost major telecom companies don't like. –  Isaac Rabinovitch May 8 '13 at 0:11

There are multiple reasons why as ISP uses DHCP (the service that hands out dynamic IPs). They dont need to change your IP. Actually, in most cases, you will get the same one when your lease (how long you keep that address, before it asks for one again) expires.

  1. It saves them money. Without going into detail, static IPs require more work to manage.
  2. It scalable. ISPs can add new customers, even more than they have public IPs for.
  3. It allows them to charge for static addresses(justifiably so... arguable)
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Point 2 should only ever apply to a modem bank. With always on ADSL, fibre or wireless links that would be dumber than dumb. –  Matt H May 2 '13 at 3:22
    
@MattH What percentage of customers would really notice if they got put on NAT? I would, because I use ssh and remote desktop. But a lot of people have NAT anyway. They could, in principle, set it up to only give them a "real" IP address if they set up port forwards... or only if they complain. –  Random832 May 2 '13 at 14:11

Simply because

  1. Its much easier to manage client IP addresses by the use of DHCP. It allows for efficient allocation of IP addresses. Only active machines are using an address.Normally for a regular user the DHCP keeps giving out the same IP address by renewing the lease.
  2. Another big reason that I can see is most of the ISP have different tariff plans for commercial connections and home connections. Static IP addresses are only given to commercial customers. They may even block port 80 traffic so that no web servers can be run.
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In the days of analogue modems, each modem was assigned the IP address and who ever dialed into the modem received that IP Address. If you dialed into the same modem, you would receive that same IP Address, if a different modem answered, your IP Address would change.

This of course could be over ridden, but it was easier just to assign a specific modem to a customer, if they wanted a permanent connection, and then their IP Address didn't change.

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ISP's change your External IP Address in order to prevent hacking and stalking from daily use on the internet. If you kept the same IP (as in a static IP), you may be able to be tracked on the internet because you always have the same address. This is okay for servers because they require a static IP in order to be found on the net in the same place every time.

Hope this helps.

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Talk about an optimist ;) If anything ISPs would be encouraged by LEA to try to keep IPs the same even for dynamic IP customers. No one with power is trying to defend your right to privacy - they are trying to prevent many kinds of crimes. –  Ram May 7 '13 at 17:03

Another reason for dynamic ips is that fact that when the internet was first made you had much smaller ips(in the format of ivp4 and others) meaning only so many people could be on the internet at once but at that time it was ok. As internet has spread and more people are using it, there have been ivp6 and later evolutions which allow for more diffrent ips meaning more users online at once. Dynamic ips allow for ips to be asigned to someone when there are no free ips and take the ip from someone who is not on the internet. This is a much cheaper options then creating a new standard and making all the routers in the world compatible.

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