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My wireless router shows the IP address is 113.46.75.*, which some websites like ip138.com, www.ip.cn show as my IP.

But other websites, like googling for "what is my ip", www.whatismyip.com, www.whatsmyip.org, show that my IP is 121.18.126.*.

I am very curious why this happens and how is it possible.

So, why do different websites show that I have different IPs?

Thanks all!

Plus: I changed my IP address several times (restarting the modem) and all addresses is like 113.46.*.*, but my IP address does not change when googling for "what is my ip".

My ISP is a small company, so I think it is true that when visiting some websites, I am NATted or proxied to some IP addresses which have access to those websites -- not to avoid firewall though, since I am still blocked from some websites.

But I still want some more information or resources to support our guess. Thank you!

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migrated from stackoverflow.com Feb 18 '12 at 3:35

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Do a traceroute to any website and watch how many times the ip changes, gives you an idea what NAT does....en.wikipedia.org/wiki/Traceroute –  Moab Feb 18 '12 at 4:38
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3 Answers 3

up vote 3 down vote accepted

Chances are that what you are seeing as this "wrong" IP address is a transparent proxy between you and your Internet service providers connection to the internet.

This is basically what paxdiablo was trying to get across, with NAT you don't necessarily get local addresses assigned and it is actually possible that you have a fully accessible "normal" IP address (so you can run servers and voip clients that require incoming connections) but your actual data connection is through a transparent proxy that catches and caches a good number of websites so that they can do things like caching common websites and reduce overall bandwidth used for those sites.

What this means is that depending on how the site resolves your IP or whether the transparent proxy kicked in your address may appear differently to the site that is trying to trace you.

I have found this a couple of times with my isps in the past. If you try to do a 'whois' lookup on this odd ip address then you may well find that it belongs to either your isp or their upstream provider.

An alternative is a 'hostname' lookup which may resolve to something useful like proxy.somelocation.myisp.com as it has done for me in the past when I have encountered this seemingly odd situation.

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ip138.com, www.ip.cn appear to be Chinese, the other sites are not.

If you are also in China, then perhaps you are being proxied to sites outside of China to avoid government firewalls (or alike)?

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Welcome to the wonderful world of NATting, firewalls, routers and other associated network infrastructure.

Your local IP address is rarely what the outside world sees you as simply because of NAT (network address translation).

Your ISP or local router will generally do this translation for you in such a way that many different people can share the same IP address (local ones like 10.x.x.x or 192.168.x.x, for example, and from memory so don't take the actual values as gospel). These local addresses are ... well ... local, so they don't get "published" to the real Internet.

That way, we didn't run out of IPv4 addresses back in 1986 :-)

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113.46.75.* does not look like a local IP address, though. –  Thilo Feb 18 '12 at 3:32
    
No, that may be the address assigned by the ISP. It's likely to have a bank of real addresses which it can use. Or it may be a misconfigured router :-) The possibilities are almost endless. –  paxdiablo Feb 18 '12 at 3:34
    
Reserved addresses are those in the ranges 10.x.x.x, 192.168.x.x and 172.16.x.x (thru 172.31.x.x). See en.wikipedia.org/wiki/IP_address#IPv4_private_addresses –  Kyle Trauberman Feb 18 '12 at 3:35
    
No, 113.46.75.* is not local. So my ISP assigns another ip address for me when visiting some websites and translate it back when receiving the response? seems complex. –  saltycookie Feb 18 '12 at 3:43
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Ummm. Local and global ips have nothing to do here. In both cases the websites are showing global ips. And those are not guess, they are well defined. –  Bibhas Feb 18 '12 at 6:44
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