People who use high-speed connections, such as cable modems at home, might be prone to hackers' attacks because computers connected to the Internet in this way are more vulnerable and more enticing to the hackers. To protect home computers, many people have turned to personal firewallssoftware that runs on the computer and protects the computer against Internet attacks. To understand how personal firewalls work, you first need to understand the concept of Internet ports. An Internet port isn't a physical devicerather it's a virtual entranceway between your computer and the Internet. When you make an Internet connection, many of these virtual connections are opened up, and each has its own number and purpose. For example, email software usually uses port 110 on a mail server to get mail and uses port 25 on a mail server to send mail. FTP software usually connects to FTP servers using port 21.