Network security has become an important part of today's IT staffs. However, there is a small part of it that needs to be a part of everybody's understanding that works with computers that attach to the Internet. I will review some basic ways to inventory your systems externally. Basically, this will allow you to become more informed while enabling you to check on the work that your network administrator performs. Networking openings are the most obvious way that external agents can get into your system. Here's an overview on how to check your systems for openings from a different machine on a network. In fact, you can check any machine (or the firewall that protects them) on the Internet. This will let you know how concerned you should be about new security leaks and which software patches may apply to your network. For example, suppose you have a Sun Solaris Web server running the intranet on your network. When you check the machine, you find out that the print server installed by Solaris is also running on it. This is true even though there is no printer connected, and it's only a Web server -- it was installed by default when the computer was set up. In the best case, you receive the CERT advisory CA-2001-15. This will show you that you may be vulnerable to an attack because the print server is turned on. In the worst case, you don't know about the print server and someone scans your network ports. They attack your machine, and now own all the data on your intranet. To see what may be listening on the computers in your network, you should use a simple hacker's tool known as a port scanner. Software used across a network listens to network information on a port. There are a number of ports available on most servers. By using a tool known as a port scanner, a hacker checks for every possible piece of network software. If it answers, the hacker tries to find more information about the computer. The hacker then tries to exploit that port. However, you can use it just as a list of what's listening on a computer and check to make sure you don't have unnecessary software running. Here are a few port scanners that you can use.