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On ISP level, is it commonly limited to some subnet mask? Is it possible or rare for an ISP to randomly change an IP's first 2 bytes?

Just as a bit of backstory, I'm trying to deter users from connecting with multiple, drastically different IPs, if anyone knows a better way to do this it is much appreciated. I have attempted using geolocation databases but they are expensive and not quite as accurate as I had hoped.


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What's the actual problem you're trying to solve?? – Chris S Aug 31 '12 at 15:58
up vote 1 down vote accepted

Some ISPs have different ip ranges. My home isp for example has range in the range and and a couple other which I cannot remember from the top of my head.

So yes, it is possible to have different IPs from different ranges, since ISPs got them at different times, and different pools were free at the time.

AFAIK, RIRs cannot get smaller ranges then /8, so should(!) all be in Europe (assigned by RIPE).

You can always check the whois database to see which ISP is the owner of that IP.

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Very helpful. Would it be reliable to use the CIDR obtained from the whois? – Whired Aug 31 '12 at 16:00
Problem is due to ipv4 depletion, IP's have been sold and quite probably will be traded even more meaning they may not match original geographic locations. whois should work though. – Robin Gill Aug 31 '12 at 16:04
whois should be good enough, since it has to be updated to check IP-range <-> AS number, but geographic location is very limited (sometimes even with a wrong continent in case of anycast - like googles dns address) – mulaz Aug 31 '12 at 16:10

It is possible and not rare at all (of course, it will not change randomly, ISP can have multiple /but limited number of/ IP address blocks).

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I assume you are asking about consumer grade broadband, or other services without a static IP.

The definitive answer will vary from provider to provider, but I know for a fact my home changes significantly.

Just some examples of recent IP's my home router has picked up via DHCP from the ISP:


I've had literally dozens of different addresses for each of those - e.g.,,,, etc.

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Significantly being more than the last 16 bits? – Whired Aug 31 '12 at 15:53
I'm sure there are another few ranges my ISP uses, but I've got these from access logs on a forum I moderate and some of those I haven't listed could have been from usage from places other than home so I didn't provide the lot. (Also would have been a massive post). – Robin Gill Aug 31 '12 at 16:02

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