I thought I knew TCP & UDP/IP but I was reminded this week about the 188.8.131.52/4 multicast subnet. If you're ever in a position where you need to identify a device's IP address (even on a different subnet, but the same LAN segment) you can PING 184.108.40.206 and everything on the segment will respond to the PING (firewall settings permitting).
So, if I set my machine's IP address to 192.168.1.220 on a 10.100.100.x network and then PING the multicast address;
You can see that all the machines on the 10.100.100.0/8 network respond.
This comes in very useful with Amulet DXiP cards which you configure over a web interface. Our demo kit came back from a customer who had forgotten what they had hard-set the cards' IP addresses to and this technique was a life-saver.
Thanks for reminding me of this Graham and Don Poves!