How do I know a device's IP address after DHCP has assigned it?

I plug an instrument, an adapter, or some other little ethernet enabled device into a network with DHCP. A host assigns it an IP address. Now I want to telnet to it, or ftp to it, or open its web page to configure it. How do I find its IP address to do that? It’s just a little box with no buttons or display.


Check the DHCP server active client list.

Maybe this thread over on Tech Support Guy has what you need?

How to find the IP address of a device

Thanks - how would I do that? I’m sitting on a big network I don’t manage…

You’ll need help of the network admins to get to the DHCP client list. Otherwise it will just be trial and error.

Most little boxes don’t work that way.
They default to a particular IP address at power-up, and then you configure them to use an assigned IP address. You tell the router to reserve a range of IP addresses that will ever be assigned by DHCP, and you put all your little boxes in this address range. That way you are never going to lose communication with your little box when its’ lease is up.

If you are on the same network segment, use arp to get a list of MAC address to IP address values. You should be able to find out the mac address of your device (it will probably be printed on the device somewhere).


Bee-wolf, half the little boxes on my desk work that way. I chalk it up to luck. I have my home network set up the way you describe, but for some reason at work they don’t do it that way. Instead there are subnets that are all DHCP and subnets that are all static - well, as I understand it. You may remember my recent casting about for workarounds…

si_blakely, this is interesting and I experimented with it some. Thanks. If I had known to ask for such a thing a few hours ago, it would have saved a right fine bit of effort. Handy!

There’s tools like Angry Scanner that will ping all the IPs in a range. Find the one with the MAC address of your device, and ta-da.
The first 6 hex digits of the 12-digit MAC address are the manufacturer code. Google ethernet manufacturer codes and see if the suspected MAC code matches who made you box. If you can’t plug the device into a tame network first to determine the MAC address (as mentioned above) then if it’s only one of a few such devices on the network, you can try all of them. Most devices list the MAC somewhere on the box.

Or let’s say you think you know the address but want to be sure. Then ping the address continuous, i.e. in a DOS command box run command > PING -t (where is the address). The device will ping continuously. Unplug the network, ping should time out. Plug back in and it resumes.