Your modem may well use nat or similar. Nat restricts most incoming ports. NAT stands for network address translation, a system which lets you share an Internet connection among many computers while allowing for unique IP addresses for each computer. Many DSL and Cable modem switches use NAT. Apple's Airport uses NAT. If you're using a switch box to share your internet connection with many computers or are accessing the internet via a wireless connection, you may be using NAT and will not be able to share files. At least with modems/routers you can use port forwarding which opens up ports for particular purposes usch as gaming, video conferencing, p2p use, etc.