Today so many people are hidden behind firewalls, this is not a real problem most of the time, but it becomes annoying when people who run a fserve use different listening ports.
A solution is to use uPNP feature of all the recent firewalls (old ones with firmware upgrade too), that way you can play like if you were firewall-less.
This is a protocol that dynamycally open a listening port on the WAN so that an application behind can accept an incoming connexion.
It's totally secure from the virus point of view because you can also set up a password for this feature.
Better than all, the port is only open when you need it: no more forwarded port to listening application that a hacker/virus can detect on your computer.
