A firewall protects your network from unauthorized trespassers.
Many routers have firewall functions built in to check traffic leaving or entering the network, but you also should run a personal firewall on each client computer on your WLAN.
Personal firewall software usually costs around $40 and it is often bundled with antivirus software.
A personal firewall is an application that runs on your computer and protects it against unauthorized access.
Like a hardware firewall, it’s configurable to allow or disallow different types of traffic to enter and leave your PC.
Personal firewalls monitor ports, which are numbered software addresses that your computer uses for different networking tasks.
For example, a Web server communicates through port 80, an e-mail server through port 25.
Many of these ports aren’t needed by your PC, and a good personal firewall closes or blocks unused or unneeded ports to prevent an intruder from accessing your computer through one of them.
A personal firewall also can monitor activity on your PC and alert you when an application attempts to reach the Internet or your WLAN.
This is important because some worms and Trojan horse software attempt to use your computer to infect other computers, or to “phone home” to a cracker so that he can get into your computer.
Personal firewalls are an effective way to protect individual computers on your WLAN.