topic Re: Two NICs with the same IP in a PC? in Communications and Wireless

Two NICs with the same IP in a PC?

F.J.Llorente Wayfarer — Mon, 19 Apr 2004 04:54:27 GMT

Hello everyone!

1.- The Environment: I have one PC with Windows 2000. It has two NICs attached. I need to connect one of the NICs to the company's network, and the other one, via a crossover ethernet cable, to my laptop. So, both NICs will be attached to different physical networks.

2.- The Problem: The laptop have an IP in the same subnet of the PC, and I should not change it, as that laptop is also from the company.

3.- The question: Could I assign the same IP to both NICs in the PC? Should I create any static routes? I think at least one pointing to the IP of the laptop should be necessary...

Any idea will be appreciated. Thanks in advance!

-- Wayfarer

Re: Two NICs with the same IP in a PC?

Ernest Ford — Mon, 19 Apr 2004 22:09:16 GMT

I don't think the OS is going to allow you to set the same IP on both NICs, it also may not allow you to have IPs in the same subnet and even if you could, you can't route within the same subnet.

What I don't understand is why you don't just simply connect the PC and the laptop to the LAN at the same time - they both have IPs in the same subnet it should work

Re: Two NICs with the same IP in a PC?

John Collier — Tue, 20 Apr 2004 18:48:05 GMT

Wayfarer,

I have bad news for you. Having two identical IP addresses on the same network will NOT work no matter how badly you want it to. Two NIC cards in the same machine with the same IP address will cause M$ Windows to shut them both down and basically render them useless.

The best solution to this is actually DHCP from your company network. Are you sure that the company doesn't use DHCP to assign IP addresses? Have you checked the network configuration on your machines to verify this one way or the other?

If they do, then the only NIC configuration that you will need to worry about is the one that you plan on using the crossover connection on. You can static that one to whatever IP you want and then static the IP on the laptop to one right above it as long as you have the two of them directly connected and then change the laptop back to look for the company's DHCP servers when you are through with the direct connect.

I know that isn't the smoothest way to accomplish your goals, but it will work.

Let us know,
John

Re: Two NICs with the same IP in a PC?

Peter Serwe_1 — Wed, 21 Apr 2004 11:56:00 GMT

There's an easy solution to this problem.

One interface #1 of the PC, connect it
to the corporate lan. On interface #2
of the PC, connect it to the crossover
cable. Set up a private network number
alias on interface 2 of the PC. I.E.:

10.0.0.2, or 192.168.0.2, or 172.16.0.2.

Don't configure a gateway address, because
that IP address doesn't need to connect to
the outside world.

On the interface of the laptop, add an IP
alias address of 10.0.0.3, 192.168.0.3, or
172.16.0.3. Once again, don't configure a
gateway for that network. Put a subnet mask
of whatever you want, although 255.255.255.0
should probably be the easiest to avoid any
other network conflicts.

Connect the crossover cable between interface
2 on the PC, and the laptop's single
interface. Ping from the laptop to whatever
the PC's interface is. Repeat from the PC
to the laptop. As long as they are on the
same network with the same subnet mask, they
should be able to ping eachother, and
TCP/IP, as long you can ping, is working as
it should be.

I have two burning questions that came to
mind as I re-read your initial post.

1) Why do you need to bypass the company's
network? Do you not have 2 jacks in the
office?

2) Is the network so congested that your
wanting to bypass it? Or are you just
trying to keep traffic off the corp lan?

Re: Two NICs with the same IP in a PC?

F.J.Llorente Wayfarer — Thu, 22 Apr 2004 05:26:07 GMT

Peter:

1) Yeah, we do not have two jacks...

2) Yeah, I need to transfer a lot of data from the laptop to the PC, because I'm doing some backups...

Btw, a workaround like the one yo have proposeed have worked fine. I just assigned to the second NIC another IP address of the same subnet... That way I've been able to connect to the PC and transfer all the data, without need to modify the laptop configuration, wich at last was one of the conditions.

So, the problem is solved so far. Thanks to everyone!

-- Wayfarer