First a definition.....

"A Firewall is method of guarding a private network by analyzing the data leaving and entering. Firewalls can also provide network address translation, so the IP addresses of computers inside the firewall stay hidden from view. Packet-filtering firewalls use rules based on a packet’s source, destination, port or other basic information to determine whether or not to allow it into the network. More advanced stateful packet filtering firewalls have access to more information from which to make their decisions. Proxy firewalls, which look at content and can involve authentication and encryption, can be more flexible and secure but also tend to be far slower. Although firewalls are difficult to configure correctly, security experts generally agree that they are a critical component of network security."

OK, so what this mean to me? Both Windows and Macintosh operating systems come with a built firewall. In most cases, this should be configured to block all incoming port traffic. This will not preclude you from using a web browser or checking email. Many of the attacks to personal computers are done through various ports that a computer uses to communicate with. The firewall allows you to deny all access or specify which ports you might want open. For instance, if you need to run a web server (not a good idea on your home computer), you want need to allow access for port 80 which is what web browsers need to communicate to a web server. Although the operating systems do have a firewall built into them now, you might consider using a third party firewall such as Norton Personal Firewall if you are going to need to have open ports and you want better control of your configuration.

Users of broadband Internet connections may have a router of some type which provides a hardware firewall of sort. Using DHCP and NAT, your local computer would be invisible to the Internet. You can then configure it to do "port forwarding".

The bottom line is that unless you have a very good reason to do so, you should have all port access to your computer blocked or you are vulnerable to attack.

Symantec's web site has a "Vulnerability Checker" that you can use to determine whether you are adequately protected or not.