What Is a Firewall and How Does a Firewall Work?

A firewall is the first line of defense protecting your network

Computer lock representing a firewall
Photo: PhotoAlto / Gabriel Sanchez / Getty

As you learn the essentials of computer and network security, you will encounter many new terms: encryption, port, Trojan and others. Firewall is a term that will appear again and again. 

What Is a Firewall?

A firewall is the first line of defense for your network. The basic purpose of a firewall is to keep uninvited guests from browsing your network. A firewall can be a hardware device or a software application that is usually positioned at the perimeter of the network to act as the gatekeeper for all incoming and outgoing traffic.

A firewall allows you to establish certain rules to identify the traffic that should be allowed in or out of your private network. Depending on the type of firewall that is implemented, you can restrict access to only certain IP addresses and domain names or you can block certain types of traffic by blocking the TCP/IP ports they use.

How Does a Firewall Work?

There are basically four mechanisms used by firewalls to restrict traffic. One device or application may use more than one of these to provide in-depth protection. The four mechanisms are packet filtering, circuit-level gateway, proxy server, and application gateway.

Packet Filtering

packet filter intercepts all traffic to and from the network and evaluates it against the rules you provide. Typically the packet filter can assess the source IP address, source port, destination IP address, and destination port. It is these criteria that you can filter to allow or disallow traffic from certain IP addresses or on certain ports.

Circuit-Level Gateway

A circuit-level gateway blocks all incoming traffic to any host but itself. Internally, the client machines run software to allow them to establish a connection with the circuit-level gateway machine. To the outside world, it appears that all communication from your internal network is originating from the circuit-level gateway.

Proxy Server

proxy server is generally put in place to boost performance of the network, but it can act as a sort of firewall as well. Proxy servers hide your internal addresses so that all communications appear to originate from the proxy server itself. A proxy server caches pages that have been requested. If User A goes to Yahoo.com, the proxy server sends the request to Yahoo.com and retrieves the webpage. If User B then connects to Yahoo.com, the proxy server just sends the information it already retrieved for User A so it is returned much faster than having to get it from Yahoo.com again. You can configure a proxy server to block access to certain websites and filter certain port traffic to protect your internal network.

Application Gateway

An application gateway is essentially another sort of proxy server. The internal client first establishes a connection with the application gateway. The application gateway determines if the connection should be allowed or not and then establishes a connection with the destination computer. All communications go through two connections—client to application gateway and application gateway to destination. The application gateway monitors all traffic against its rules before deciding whether to forward it.

As with the other proxy server types, the application gateway is the only address seen by the outside world so the internal network is protected.

Note: This legacy article was edited by Andy O'Donnell