On a cisco switch, enable, and show mac-address-table to find out which host(s) are connected to which ports.
If you know the port (for example, this evening I need port fa0/22), do a sh mac-address-table interface fastEthernet 0/22
Then, from a host with decent broadcast ping, do a broadcast ping, and grep -i for the mac address from the output of arp -a.
Oh, so *that’s* the host that is going overboard, eh? Gotcha.
Now, all I have to do is search my blog for cisco, and I’ll have the answer. Quickly.