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IP addresses are encoded into email headers. But if you get two emails from the same IP address you can't assume that they came from the same computer.

I have Hotmail and I've been getting nasty e-mails from somebody who I do not know. I figured out how to view the headers and try trace the IP addresses. As I was doing some trial and error from the X-Originating-IP addresses from people on my list, I noticed that one of my friends has the EXACT same X-Originating-IP address from the one I've been getting my nasty e-mails from. Is my old friend sending me nasty e-mails off of the same computer but through different e-mails? If it helps, there both Hotmail accounts. Thank you in advance for your help and assistance.

Of course he could be, but the IP address doesn't prove it.

There are several reasons that a single IP address could be used by several different computers.

In the simplest case, an IP address uniquely identifies your computer on the internet. However for many reasons that's becoming less and less common as computers proliferate.


An IP address only identifies whatever it is you have connected to the internet. In many cases these days, that's a router:

Router connecting several machines to the internet

In a case such as this, all the computers to the left of the router will appear on the internet as having the same IP address. That IP address is actually assigned to the router, and it handles routing the traffic to the appropriate computer on the local network.

In a case like this the IP address you've extracted from your email headers may get you as far as the router, but that's it. You can't tell which computer behind the router was responsible for it.

The diagram above is a common home or small business configuration. It's important to realize, though, that in larger installations there could easily be hundreds of computers sharing a single or smaller set of IP addresses. Once again, with just the internet IP address, there's no way to tell which computer sent your email.

Dynamic IP Addresses

Many computers are connected to the internet using what's called a "dynamic" IP address. The IP address is assigned to that computer when it first connects to the internet, and is released when it disconnects. A common example is dial-up connectivity where the connection and disconnection are both obvious and frequent. Persistent connections can also use dynamic IP addresses, and in fact can be re-assigned a new address even without having to disconnect - though typically that's not the case. However even the slightest disconnection could cause a new IP address to be assigned.

" larger installations there could easily be hundreds of computers sharing a single or smaller set of IP addresses."

What's important to note here is that the IP address you were assigned yesterday might very well be used by someone else today.

That means if your sender is using a dynamic IP address, then it might be someone else entirely if you see that same IP address in another email at a later time. There's no obvious way to know.

Local IP addresses

If the address you see begins with 192.168., 172.16. through 172.32. or 10. then it's not an internet IP address at all, but rather a local IP address assigned by a router.

Looking at the diagram above again, you can see that internet IP addresses are assigned to the router's connection to the internet. However on the left, on the local side of the router, the addresses are assigned from a range of IPs reserved for local networks. Most home and small business routers assign from the 192.168. address range.

The problem here is that if that's the IP address you're seeing, then it tells you pretty much nothing. There are probably tens of thousands of machines with that 192.168.?.? IP address, scattered on local networks around the internet.

Without the internet IP address, there's just no way to get closer.

The Bottom Line

Ultimately, as I've said time and time again, trying to use IP addresses to locate someone is futile for the average person. Yes, technically there may be ways to backtrack, but it's complex, and often involves breaching privacy barriers that will require law enforcement and/or court orders.

Article C2931 - February 12, 2007 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

kelly pettit
February 24, 2007 12:37 AM

I'm in a battle of the bands compitition where the winner is judged by the amount of votes they get on line. You can vote once a day from your computer. They count your IP address. I'm worried that if an office of 40 friends all vote for me, they only count as one vote if it's the same IP address. 40 different computers, 40 people voting, one vote counted. Can this happen?

Leo Notenboom
February 24, 2007 10:38 AM

Yes, it could.

Eli Coten
February 24, 2007 12:50 PM

The crucial point in this case is that it's being sent from Hotmail. Any emails sent through a web-based service will have the webmail provider's email address as the X-Originating-IP because the actual computer used to send the message is not writing the headers. The headers will be written by the server-side script on the Webmail hosting providers service.

So any email sent from a webmail service will have one of the IP addresses that resolves back to the webmail provider, and all you can deduce from it is that the spammer was connected to the same webmail server as your friend on hotmail.

Leo Notenboom
February 25, 2007 3:37 PM

Hash: SHA1

Actually, HotMail does occasionally include IP address of the computer
accessing the web service as the x-Originating-IP - I just confirmed it.
Not sure about other providers, but that bit of anonymity obviously
isn't guaranteed.

Version: GnuPG v1.4.6 (MingW32)


January 11, 2009 3:20 PM does include at least the IP address of the router that the user was connected too, I have confirmed that when i send an email through hotmail it includes my routers IP every time without fail.

It is possible your friend sent the mails but you cant be sure, what you could do is check all the IP's of your friend and also from the person sending the malicious emails and see if there is any pattern.

January 13, 2010 11:51 PM

I'm having the exact same issue. I've compared the malicious emails with the other emails and found that one of the IPs found in both email addresses headers is EXACTLY the same (starts with 10. and the rest of the numbers match identically). Does this prove anything?

Not really, though it might be useful with other data. All that a "10." address means is that the person is behind a router. That exact same "10." address could exist behind hundreds if not thousands of different routers.

John Heaney
March 2, 2010 8:12 AM

Hi Leo - your site and info has been most helpful. I think I have the answer but hopefully you can help. I have had a number of e-mail from two different people supposedly, but both mails have come from IP Identical. These emails have come to me over a period of about 2 weeks. These MUST be coming from the same computer, or router. Am I right? So one e-mail couldn't be coming from Aberdeen, and one from London.

Many thanks,

John Heaney

You simply can't make that assumption. ISPs do funny things with IP addresses, particularly if they IP addresses are assigned dynamically. Not only could that single IP address have been assigned to different computers over time, but there's no guarantee that the computers are near each other. Similarly, if the IP address is of a NAT router, such as you and I might use in the home, then there could be any number of computers sharing that same IP address.

April 5, 2010 9:55 PM

I am getting abuse emails showing my close friends Static IP address.. But i gotta doubt on my new roomate. Is it possible for my roomate to use my Friend's static IP and send abuse emails to me using our common internet connection.
[Note: we use same internet connection and same computer for internet perpose..]
Pls help me with the solution... thanks!

October 1, 2012 9:25 AM

Hi Sir,
My email was compromised and I am sure it was my sister in law. I checked the IP who logged in my account and was and her emails come from IP which is How can I prove that to Immigration office? Please advice. She is ruining my immigration. Please advice. Thank you very much.

Mark J
October 1, 2012 9:43 AM

That is more a question for a lawyer with experience in IT cases, than a tech website with no expertise in legal matters, unless there is a lawyer out there who chooses to weigh in.

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