A firewall is a hardware or software solution that protects your computer network by preventing its access by unauthorised or potentially dangerous software, by using a system of built-in filters.



A firewall is typically a software package, or a dedicated gateway machine. The gateway acts as a security system for networks connected to the Internet. It is a doorway between a user on the network and the Internet. A firewall lets you use all your regular Internet features – email, chat, web browsing, etc., while making sure other Internet users cannot access your computer.

What does a firewall do?

A firewall can protect a computer network in many ways:

• It can screen out incoming viruses, spyware and adware.

• It can be set up to disallow access to Internet content. Large organisations normally use a firewall to disable access to content like MP3s, pornography, online games, online instant messengers, etc.

• It protects a network from access by other networks.

• It allows users with only the right access codes/passwords to access the network. Thus, it blocks hackers and crackers from accessing the network.

• It can cache web sites that have been previously visited, thus avoiding excessive use of bandwidth.

How does a firewall work?

Firewalls are placed at the junction of a network and the Internet. It then screens packets of information passing through it, and decides whether the packet is authorised or not.

There are two access denial methods used by firewalls. A firewall can either allow all traffic through unless it meets certain criteria, or deny all traffic until it meets certain criteria. Firewalls usually screen the type of traffic, or the source and destination of the traffic.

Types of firewalls

Firewalls can be both software and hardware. There are five basic types of firewalls:

• Packet filtering firewalls: These firewalls are usually a part of the network router. Each packet passing through this firewall is screened and compared to an administrator-defined set of criteria. Depending upon the criteria, the packet is either forwarded to its destination or dropped from the network. These firewalls don't cost much, and don't slow down network performance.

• Network Address Translation (NAT) firewalls: NAT routers do all that packet filtering firewalls do. In addition, they hide the IP addresses of computers on the network.

• Circuit level gateways: These firewalls do not filter individual packets. Rather, they monitor TCP handshaking between packets. What's more, each packet that passes through a circuit level gateway appears to have originated from the gateway and not the computers behind it. Thus, network computers stay protected. These gateways are relatively low-cost.

• Application level gateways: These gateways are also called proxy servers or proxies. They are actually application-specific circuit-level gateways. These gateways don't allow any ftp, gopher and telnet traffic through. Proxies are configured to only let through http:post, get and other specific commands through. Application level gateways can also monitor user log-ins and log-outs, as well as user activity. They are very secure, but significantly slow down network performance.

• Stateful multilayer inspection firewall: These firewalls combine the other four types of firewalls. They screen packets, determine their legitimacy and evaluate contents of packets at the application level. They are highly secure, very expensive and need to be administered by highly competent network personnel.