[TZubiri]

>All home routers block all ports by default. How would they know which IP and port to forward traffic to if not for manual configuration?

I might have to recommend brushing up on your IP/TCP

This is what routers do by default. IP is very different from IP. IP is what they use to know how to route, too long to fit in a post, but MAC to IPs form a local ARP table. Can be many nodes long. Ports are what identify a process in a machine, IP level routers don't care at all about the port (unless you are natting) in order to route, the port bytes are forwarded as is and only parsed by the destination (and constructed by the source)