We have a router with that acts as gateway, with its own IP.
We have 4 computers connected to the router, on a subnet.
We have 4 IPs on the subnet.
One of the IPs cannot be used. The router thinks it is taken,
and gives a hardware address that corresponds to none of the
computers.
None of the computers believe that they have the IP.

Can you help free up the IP?

Hi there!

Thanks for your question. You can try to free up the IP address by
reconfiguring your router, or by renewing IP addresses one or all of
your computers.

To reconfigure your router:
Usually a router/firewall/gateway such as the Linksys Instant
Broadband™ EtherFast® Cable/DSL Router
(http://www.linksys.com/Products/product.asp?grid=23&prid=20) allows
you to configure the IP addresses by logging into it via a web browser
(usually by typing in http://192.168.1.1). See page 36 of their user's
manual (ftp://ftp.linksys.com/pdf/befsr11_befsr41ug.pdf). If you are
reconfiguring, you can set the IP addresses to start at a new subnet
#. Then you can restart all your computers so that the DHCP server
(router) assigns the new IP addresses.

You might want to consult your hardware manual for your particular
router controls/configuration settings.

To renew your DHCP-assigned IP address on a computer (assuming Windows
95/98/NT/2k/XP):
1) Click Start->Run-> 
2) Type in "command" (no quotes) to pop up a MS-DOS window
3) At the C:\> prompt, type ipconfig \all           (this shows you
the current IP address of the computer and when it was
assigned/expires)
4) Type ipconfig \renew           (this sends a request to the DHCP
server -- your router/gateway -- to give it a new address).

This Microsoft Knowledgebase article describes IPCONFIG in a little
more detail (http://support.microsoft.com/default.aspx?scid=kb;en-us;Q117662)

I think that one of these methods should help free up and reassign the
IP address. If not, please provide the following information:
1) Operating system (Windows 95/98/NT/2k/XP, Linux, etc.)
2) Brand and model of router (e.g., Linksys BEFSR41- Cable/DSL Router
with 4 port Switch)
3) results from IPCONFIG \all on suspect computer

Thanks for using GoogleAnswers!

drunker-ga was the most helpful, by suggesting that the router may
have snatched the IP.
In fact, that was the case! I reconfigured the router and the IP was
freed up.
We do not use DHCP, but when the router went through a cold re-start
it must have defaulted to DHCP support(?) Either that, or by some
other means
the IP was being held by the router for its own purposes.
Thanks for being there!

If you post more specifics on the hardware you are using I may be able
to give you a better answer.

Personally I would not look to the clients for answers until the
address is removed once and shows up again.

First step if possible is to reset the router, that should clear it's
memory, then if you still have a problem follow the above instructions
for reseting client IP addresses.

The correct command line is:

ipconfig /all

using / and not \

Also, you might find that if you are using an older Windows operating
system the command is "winipconfig" which will bring up a dialog box
with the same information and also buttons to release and renew as
described by mit!

Hope mit's info helps you out!

Could it be possible that the router is reserving the address for its
own DHCP server? Comments welcome.

Normally in a DHCP environment, the router will be at 10.0.0.1, and
you can add additional computers at 10.0.0.2, 10.0.0.3, etc.  I've had
a few occasions where I reboot, and for some reason it will not
release 10.0.0.2.  Try making more IPs available, something like
10.0.0.1-10.0.0.9.  You don't have to worry about anyone from the
Internet using the extra IPs, as the 10-block, and 168.147.x.x are
non-routable.

You could also try reserving the hardware addresses of your other
computers in the router.  To find the hardware addresses of remote
computers in Windows, use PING 10.0.0.2 (or the correct IP), then ARP
-a to display the hardware addresses.

If you would like a good site on Subnetting, try
http://www.learntosubnet.com/

Normally in a DHCP environment, the router will be at 10.0.0.1, and
you can add additional computers at 10.0.0.2, 10.0.0.3, etc.  I've had
a few occasions where I reboot, and for some reason it will not
release 10.0.0.2.  Try making more IPs available, something like
10.0.0.1-10.0.0.9.  You don't have to worry about anyone from the
Internet using the extra IPs, as the 10-block, and 168.147.x.x are
non-routable.

You could also try reserving the hardware addresses of your other
computers in the router.  To find the hardware addresses of remote
computers in Windows, use PING 10.0.0.2 (or the correct IP), then ARP
-a to display the hardware addresses.

If you would like a good site on Subnetting, try
http://www.learntosubnet.com/

Seems you might also simply be mistaking the "We have a router with
that acts as gateway, with its own IP."  You may be mistaking the fact
that the router needs two addresses, one external and one internal. 
usualy the dhcp server will also be on this ip...

If this is not it look at the first four or six digits in hex of the
MAC address...  there are plenty of MAC address registrys.  They are
assigned in blocks to manufacturers...  this way you can figure out
who mfg'd the card that has the address...

We are not using DHCP, so renew/release with ipconfig does not solve
the problem.
Operating system: Windows NT (2), Windows 98(1), Mac OS9 (1) = total 4
computers on subnet.
Router: Netopia R7100C SDSL
As for resetting the router, how will it know which IPs it has to
assign after I clear its memory?
I