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Do Internet Providers give the same IP for more than one customer?

I want to know if an ISP could give the same public IP address to more than one customer. Theoretically it is possible right?

If they can is it something they do regularly? I mean, not necessarily all the time, but I wonder if it's something that eventually happens

Me and my neighbor could be sharing the same IP to the internet?

My question actually comes from a security reason. To defend from DOS attackers I would ban their IPs for some time. Would other people get affected by that?

In a real production scenario, would you solve DOS by blocking IPs at application layer?

Victor Ferreira
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  • i really don't think so. maybe try 'security exchange'? – Dr Upvote Feb 23 '17 at 03:03
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    How is this a programming question? At best, it's a [su] question. At worst, it's an opinion poll (*if you were me would you do that?*), which is entirely off-topic here. – Ken White Feb 23 '17 at 03:04
  • this question is in the middle of programming and platforms I believe. I will implement a security feature in my application as explained in the last part. so I need to know how this decision will affect my system – Victor Ferreira Feb 23 '17 at 03:08
  • No, this asks nothing about programming (code) or programmers tools (an IDE, compiler, use of an API, etc.). It's at best a [su] question. (And editing to add *at application layer* to your **opinion request** does not make it on topic here.) – Ken White Feb 23 '17 at 03:09
  • programming is not code only, Ken (btw I edited the last part of the question because it wasn't very clear) – Victor Ferreira Feb 23 '17 at 03:10
  • I am looking for a solution. this solution involves making a decision that is exactly what is written in the last paragraph. is it what the community does to prevent attacks? cage gave a good answer, maybe it belongs to security exchange. but definitly not to Super User or an 'opinion request' – Victor Ferreira Feb 23 '17 at 03:13

1 Answers1

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A public IP address is normally assigned to an internet connection. Typically that means an entire home or office or an individual mobile device. So you and your neighbor will typically (unless your neighbor is using your WiFi or vice versa) be on different IP addresses.

However, many ISPs will recycle IP addresses. This happens frequently on mobile devices as the IP address at any moment will typically be assigned based on the cell tower you are connecting to. With cable, DSL and fiber connections your IP address may be constant for days or even months at a time, even without having a contracted static IP address.

Where this really comes into play is blocking spam. Many spam filters depend in whole or in part on real-time blacklists (RBLs). I have also done similar things (aha! it IS a programming question!) to block spam robots from some customer web sites.

If an IP address is associated with a region of the world where you do not expect to have many customers (or to receive any email from that region) then it is generally safe to block a large range of IP addresses. On the other hand, if the IP address is local to your customer base then blocking a large range would not be advisable, and even blocking one known problem IP address at a time can be problematic if either the IP address is reused and then effectively unavailable to some other potential customer OR if the IP address is actually an email server used by a large group of customers (this happens frequently with hosting providers that don't manage blacklist issues).

manassehkatz-Moving 2 Codidact
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  • Don't forget that there are not enough IPv4 addresses to let all ISPs give a dedicated address to each customer anymore. Many IPv4 addresses will be shared between groups of customers, so blocking one address can affect hundreds of users. – Sander Steffann Feb 25 '17 at 16:55
  • That is definitely true. But IP addresses normally have a geographic location associated with them. If based on that location you know the traffic is of no significant use for your web site then blocking the traffic by IP address can be very effective at stopping unwanted traffic. – manassehkatz-Moving 2 Codidact Feb 26 '17 at 00:50
  • As long as you realise that the scope of the geographic location is shifting from user to country level :) – Sander Steffann Feb 26 '17 at 09:28
  • Correct. I would be extremely careful with automation of IP-based blocking for exactly that reason. And in fact that is part of the problem with some IP-based spam RBLs. – manassehkatz-Moving 2 Codidact Feb 26 '17 at 15:52