A firewall is a program that protects your resources by filtering network packets. Firewalls can be run as part of another piece of software. For example, your Wi-Fi access point/router almost certainly provides some kind of firewall capabilities. Firewalls can also be run as individual programs on computers. Finally, sophisticated firewalls can be run on servers dedicated to that purpose, although this generally only happens in enterprise-class setups.
Firewalls enable a network administrator to determine which clients inside a network can access network resources and which ports can be used from outside the network to access the network. In case you are wondering, a network port is a logical endpoint on the network. The port number identifies the kind of traffic that uses the port. For example, port 80 is used to connect to a Web server using the HTTP protocol.
Effectively, firewalls can be used to isolate portions of a network topology from the rest of the network and from the Internet. This is another way of saying that you can use a firewall to limit access both to and from the Internet.
Within a wireless access point, IP filtering can be used as a gateway to control access to your private network. In addition, the firewall application built in to many access points (the D-Link AirPlus Firewall Rules screen is shown in Figure 18.3) can be used to let traffic (or not let traffic) pass through the access point.
Figure 18.3. The firewall built in to many access points can be used to allow or prevent traffic from passing through the device.
With most home or small office wireless routers, a network firewall is implemented automatically. For the most part, you don't need to do anything to administer it. The exception to this is that sometimes specific applications require inbound or outbound access through one of the (many) ports blocked by the firewall (for example, specialized content management servers). You can use the Firewall Rules screen to "punch a hole" in your network firewall, although obviously this is something to be done with care and only if you know what you are doing.