Hi Dragan,
You might consider the Linksys router. I've had one for over a year now and don't know how I got along without one before. It has an adeqate firewall and is easy to configure using a browser interface. You could probably pick up a four port model for under $90.00. I can't gaurantee that it would have any bearing on your current porblem, but It sure makes sharing an internet connection in a network easy. It's also very good for forwarding anything through the firewall directed to port 80, HTTP, to a linux web server. I also use mine linux box as a mail server using the sendmail program ports 25, SMPT, and port 110, POP3.