#1 What is connected to your modem (In other words, are there are other device between the modem and your computer)
#2 If so, to the last question: What is the brand and model of that other device?
#3 If not to the first question (number one), are you NATTED?
To detect this, do these steps.
a) Check the IP Address, Subnet Mask, Default Gateway and DNS Server(s) on your computer.
For example if you are on Windows XP, that would mean..
Go to Start -> Run.
Type in cmd and press enter.
In the new window, called the command prompt, type in ipconfig /all and press enter.
b) And compare that IP Address to the IP Address at http://www.whatsmyip.org/
If the IP Addresses are the same, you are not NATTED.
If they are different, you are NATTED.
#4 What software firewall is on your computer?
#5 What anti-virus software is on your computer?