Migration assistant over ethernet?

How do I get Migration assistant to work over ethernet?

The source computer is a 24" iMac from early 2009. The target computer is a brand-new 21" iMac. I have not even booted the new iMac yet. I am trying to do this in the setup dialog that occurs when you turn it on.

The new iMac is both plugged directly into my router via ethernet, and also connected to wifi.

No matter what I do, both computers just sit there spinning on the "Looking for other computers" screen.

I also tried connecting the computers directly to each other via an ethernet cable, and I checked that both my ethernet cables are good by testing them with another device.

I also made sure the firewall on the source computer is not enabled. Both computers are running Mavericks.

I can't believe this doesn't just work. I've been around computers and networks for a long time -- I'm pretty sure I'm not overlooking something obvious. I signed in to my router's admin screen (a recently purchased Cisco/Linksys) and it shows both computers connected in the DHCP table.

I am truly stuck. I don't know what else I can do. Help!