step one: disable spanning tree, because the LAN is so big it takes half an hour for a port to come up after plugging something in

step two: plug a very cheap switch into the network

step three: have someone move their desk around, and plug a spare cable into the switch at both ends

step four: spend a day doing a binary search by unplugging banks of switches to figure out where the heck all these packets are coming from.

I used to work in a place where every office on the floor was connected to the same ethernet hub. We also tested a server product that became a DHCP server if you checked the wrong box, so periodically everything would stop working until the team lead came out of his den and went to yell at the new guy.