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Conventional wisdom says two computers can't have the same IP address. Technically true but incomplete. The same IP can be used on different networks.

My Home network and my work network computers, have the same IP addresses. I want to remote to my work network with my laptop that is networked at home.

Do I have to manually change the IP addresses at home? If I do, do I also change the router stetting?.

Yes, "The Rule" is that no two computers can have the same IP address.

Except that the rule is incomplete. It's actually very common for two computers to have the same IP address. In fact, I'll bet that thousands of computers around the world have the same IP address as the very computer I'm typing on.

I'll explain how.

The full text of the rule should read like this:

No two computers can have the same IP address on the same network.

That phrase at the end makes all the difference.

The internet is one giant network of computers, and it's absolutely true that no two computers on the internet can have the same IP address. It's the IP address that tells one computer from another on a network.

"... no two computers on the internet can have the same IP address."

But if you're behind a router, you're not actually directly on the internet. You've set up a separate, local network that is connected to the internet through your router.

What that means is that the IP address of a machine on your local network is assigned separately and independently from the internet, and from all the other local networks around the world. My IP address is 192.168.1.5 on my local network, and it's likely that the same IP address is used on many other machines on many other local networks.

When you communicate to the internet from your machine, it crosses your router which performs what's called "Network Address Translation" - converting your local IP address to the internet IP address that has been assigned to the router's internet connection. Your local IP address is never seen on the internet.

So the question is if the local IP address of the machine you want to connect to is never seen on the internet ... how do you connect to it?

There are several ways, include configuring the router to route certain types of connections to a specific IP address on the local networking, through something called "port forwarding". So, to oversimplify in English, the router might be instructed: "when you get a connection request on your internet side that is for the remote desktop protocol, send that connection to IP address 192.168.1.1 on your local network side". This means that remote desktop connections can be made from the outside to the one computer the router has been instructed to send them too.

While workable, this approach has problems:

  • If you're trying to connect to your work computer, then it's the workplace router you need to configure, and you may not have access to it.

  • You probably need to take additional steps to ensure that your IP address on your local network does not change. It typically can.

  • It's easy(ish) to configure this to access one of your computers on your local network remotely. If you want to also be able to access another of your local machines, it's still possible, but slightly more difficult.

My preferred approach is to set up what's called a "virtual private network", and I happen to really like using Hamachi to do it.

A VPN creates a virtual network that ignores the entire local/internet/local structure of the physical network layout. It's as if the two (or more) machines running Hamachi were all on the same network - which, in fact they then are. VPNs do this by assigning additional IP addresses to all the machines connected to the VPN. These addresses are once again unique; no two machines can have the same IP address on the same network, and a VPN is one network.

Which solution is appropriate for you depends on your need, and to be honest, exactly what your company's IT department (or person) will let you do. I'd certainly recommend checking with them first.

Article C3634 - January 31, 2009 « »

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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?

5 Comments
Rahul Mehta
January 31, 2009 6:21 PM

Another approach is a remote log-in facility like LogMeIn. Go to www.logmein.com and open a free account. Then when you are on your workplace computer, login to your account and add that computer to it. From that point on, you will be able to access that computer from anywhere and any computer on Internet. Just log in to your account and "connect" to any computer that is added to your list.

KPTECH
February 1, 2009 2:20 PM

These days, most router manufacturers have at least one VPN router in their product line. Just purchase a router for each location and configure them to establish a secure VPN tunnel between them. I'd recommend the same mfg/model for both locations. This way you avoid potential compatibility issues. You can then access systems on both networks from either site. Instructions are provided in the user's guide and it's not too difficult.

If you work for someone else, they probably won't let you do this. In fact, most large companies prohibit connecting any device to their network without first obtaining permission from the I/T department, so please discuss with them first. However, if you run your own business, this is a simple and secure solution.

Fred Scott
February 4, 2009 9:53 AM

I would like to get VPM router on my second computer. Sure would speed things up here.

Chris
February 4, 2009 8:11 PM

Think of it like this. The internet is a Wide Area Network (WAN). Your little home network on your router is a Local Area Network (LAN). The WAN itself is just a bunch of LANs connected to each other using commercial routers. Your router at home is just adding to that WAN, but in a more secure way as it uses Network Address Translation (NAT). NAT simply protect your home computers by, not giving each computer on your network a unique WAN that everyone on the internet can see, but rather a smaller version of the bigger picture--A LAN address which ONLY the computers on your Local Area Network can see.

If you go to start -> run -> cmd -> ipconfig /all, you can see your LAN IP address which is hidden to the internet because of Network Address Translation. If you go to www.whatismyip.com, you will see your WAN address which is public on the internet. Only computers on the internet see this address and each address is unique across the WAN.

If you're at home, you can connect to your work computers using your work's WAN IP. A LAN IP will not work as it doesn't exist on the WAN....only on the separate local networks/LANs, protected by NAT routers.

Easy right? :)

David
October 11, 2010 12:26 PM

I have a Netgear Dualband Router, Gateway Desktop with Windows XP & 2 laptops with Windows 7
I get a message that reads,
Network Error
Windows has Detected an IP Address Conflict:
Anouther Computer on this network has the same IP address as this Computer. contact your network Administrator for help Resolving this Issue.
Please help me please thank you

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