The Minecraft client (when you enter "multiplayer" mode) says "Scanning LAN for Local Servers…"
What is it doing?
Here's my problem: if computer A and computer B are connected together through Switch1, then computer A can run the minecraft client and find a minecraft server on computer B. But if computer C is connected through Switch2, C cannot see the server running on B. However, C can ping B, and C can connect to the server at 10.1.10.143:64134 if this IP/port are directly entered.
So the mechanism by which clients locate servers is somehow blocked by Switch2. Is this UPnP? Or something similar?
It's a huge pain to keep track of the shifting IP addresses and shifting port numbers each time you start a minecraft server.
Best Answer
I did a bit of packet capturing on Wireshark and to the best of what I can tell, it uses an IGMPv2 packet sent to a lesser used multicast address (224.0.2.60), asking for anyone running a Minecraft server to report back. It uses a "Membership report" to do this.
For the nerds, here is the complete packet dump info:
Here is the raw packet dump:
So to answer your problem, you might have multicasts turned off on your router, or IGMP is disabled. I'm not completely familiar with the flags, but it looks like it there might be something in the Options/Router Alert field... Probably telling the router to not forward the multicast outside of the subnet, but that's just a guess.