This is not a pretty solution but this is how I did it tonight, with Linux.
1) Log into a Linux box
2) sudo the next step, or su
3) fire up wireshark
4) from the list of available capture interfaces select your ethernet card (for me eth0)
5) let it site for a bit, and collect some data
6) from the menu Statistics -> Conversation List -> IPv4
7) the window will fill up with all the IPs that have come across your interface card. (for me my network is 192.168.42.0 but the router/switch came up as 192.168.1.1)
The next steps are for if you find yourself where I was and the router/switch is on a different network then your real network
8) Since linux can have 2 IPs per network card, setup a second IP on the network that your router/switch is on.
ex. ifconfig eth0:0 192.168.1.100 netmask 255.255.255.0 up
9) Log into your router/switch (hope you remember the username and password)
10) change the IP to one that you will remember.
Mind you do not do this on a network unless either you own it (your home network) or you have written permission to do it on their network.