http://www.samspade.org

The home page of an excellent and free, suite of Internet tools. Sam Spade brings together most of the tools you need to explore the inner workings of the Internet, and play detective with probes to your firewall, and email spam. There are also many online versions of the tools on the site, plus a few extras

To understand the significance of the entries in a firewall log you need to know what the attacker was looking for. These sites give background of what many of the TCP/IP ports are used for, from which you can often deduce whether a probe presents any risk to you. In fact, what a port is generally used for allows you to make an assessment of whether a probe was an attack or just the by product of innocent Net activity

Home of BIND and DHCP. These are the open source and free programs for providing Domain Name Services (DNS) and Dynamic Host Configuration Protocol (DHCP) services on the Internet. While most home users are unlikely to run these services themselves, having some understanding of how they work (especially DNS) is vital to understand the workings of the Internet

Gibson Research Corp

Home of the useful but somewhat sensationalist Gibson Research Corporation, or Steve Gibson to his friends. Hosts a handy service for giving a quick (but by no means exhaustive) test of how a firewall is working. Has some very useful background information on securing Windows 9x based computers

Home of the free-for-personal use Tiny Personal Firewall, (TPF). TPF is an excellent product for those who want to get to grips with what firewalls and packet filters really do. It allows you to configure many of the things you can do on a fully fledged firewall and so is a useful training tool. To get the most out of it some knowledge of TCP/IP is strongly recommended.

For those who want to just make their Windows PC more secure without worrying too much about the details Zonealarm is more recommended