The Comcast cable modem (model number SMCD3GNV) is supposed to have port forwarding capabilities, but when I set everything up, the Minecraft server dynamic map wasn’t pulling up.
So, after buying another cable modem and verifying that you can indeed set up port forwarding for my network (Comcast wanted to charge me for their premium service to come out and set it up for me), I called comcast again, and spent several long periods of time being told that it can be set up, but they have to do it. Unsatisfied with this answer, I searched out the manufacturers’ phone number, and got ahold of a tech support person.
It ends up, this specific model is designed so that you can’t connect to your WAN address from the LAN.
Great job, Comcast. None of the 10+ people I talked to in your tech department knew anything about this.