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I just found something curious:

http://whatismyip.org/ says my IP address is 82.44.14.246

http://www.whatismyip.com/ says my IP address is 31.52.118.197

I'm based in the UK and behind a router doing NAT. The first address geo-ip's to my local town; the second address geo-ip's to an IP on the coast towards the south-east which is over 100 miles away.

How can this be? It appears different websites see that I (or my NAT router at least) have a different IP addresses. I thought that each device ought to appear to have a single address over the whole Internet. What kind of network activity is happening to cause this? Is it normal? Could it cause problems?

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migrated from serverfault.com Jan 25 '13 at 17:26

This question came from our site for professional system and network administrators.

    
Are you using Wifi in your house? Do you have any BT Openzones within range? If so, try repeating the experiment with your laptop's Wifi switched off and an ethernet cable plugged in instead. –  Ladadadada Jan 25 '13 at 11:19
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Could you post the traceroute output for the two hosts? –  Emyl Jan 25 '13 at 11:21
    
I second this strange behaviour which I have seen a couple of times, took notice of it but have not done any kind of further investigation. Strange also is that at other times the behaviour is completely normal ie both whatitsmyip sites report the same IP. I am NOT in the UK and yes I heavily use wireless at home. –  ank Jan 25 '13 at 11:31
    
This might be a proxy thing (you or your NET router may in theory use different proxy configurations per hostname, or it uses several proxies round-robin). –  Hagen von Eitzen Jan 25 '13 at 12:46
3  
I doubt he is using tor by accident... –  VBwhatnow Jan 25 '13 at 17:42

3 Answers 3

up vote 2 down vote accepted

Not a problem, not to worry. As long as you can ping remote hosts once per second for a minute with no more than 10% packet loss you are doing fine. The Web-based IP address detectors are subject to various errors, chiefly transparent Web proxies through which your request could pass. This is a known limitation of these services. (The geographic IP locators are also not necessarily accurate either.)

If you really want to figure it out, read on:

If you are using a WiFI router, and there is another open WiFI router nearby, your computer network manager could have changed the configuration. However, even without this, you are still subject to the proxying problem mentioned above.

Log into your local router and check the IP configuration of the WAN interface. On my D-Link DSL 2500U this configuration is on the Status->Network Statistics page. This is the IP address that my ISP assigned to the WAN interface of my modem when the modem logged in. That's the IP address, period, end of argument (unless the ISP is also NATing, which is rare. You would see a non-routed address like 10.0.X.X or 192.168.X.X).

Then check the IP address that you see in your router configuration against the results from the web sites. It is possible that neither is correct. In your case, the reply from www.whatsmyip.org, 82.44.14.246 resolves to cpc2-mort4-0-0-cust245.croy.cable.virginmedia.com.. This is probably the correct address, as Virginmedia.com is a retail supplier and the format of the name is that of a cable address - so you are using a cable modem, right? On the otherhand, 31.52.118.197, that was reported by www.whatsmyip.com resolves to host31-52-118-197.range31-52.btcentralplus.com., a BT host format address. I had the opposite result with these two sites, whatsmyip.org gave a completely wild address, and www.whatsmyip.com gave the correct address as seen from the DSL modem configuration.

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It might be the case that your ISP is caching the results from one of the pages and is just displaying old data. If you want to make sure that the request you send out is processed just for you, use https instead of http on services that allow it.

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Just to make sure: is this a home set up (as it sounds) or a corporate one? The question was migrated from serverfault...

If you are in a corporate environment I would guess this is caused by some sort of load balancing with several (two or also more) proxies. As long as you are using HTTP to "discover your (external) IP" you definitely have to consider proxies.

What are the proxy settings in your browser?

Also you might be able to determine your IP by sending an email to echo@tu-berlin.de (for example) and looking at the headers of the returned message. You need to be careful to look at the correct addresses though (your email server will be involved which should receive the message from your public IP; probably you want to look at the last (from the top, first in chronology) "Received:" header). That's just an additional piece of information.

Is it normal?

I would say in corporate environments: yes. Or at least plausible.

Could it cause problems?

Yes, all sorts of :) But I would not worry about it in corporate environments. At home it might be a bit more disturbing. But then, if you don't experience any issues, it seems to be OK, no?

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