FIREWALLS

What is a Firewall?

A computer firewall protects networked computers from intentional hostile intrusion that could compromise confidentiality or result in data corruption or denial of service. It may be a hardware device (Figure 1) or a software program (Figure 2) running on a secure host computer. In either case, it must have at least two network interfaces, one for the network it is intended to protect, and one for the network it is exposed to. A network firewall sits at the junction point or gateway between the two networks, usually a private network and a public network such as the Internet. The earliest computer firewalls were simple routers. The term "firewall" comes from the fact that by segmenting a network into different physical subnetworks, they limited the damage that could spread from one subnet to another- just like firedoors or firewalls.

Figure 1: Hardware Firewall

Hardware firewall providing protection to a Local Network

Figure 2: Computer with Firewall Software

Computer running firewall software to provide protection

What does a firewall do?

An Internet firewall examines all traffic routed between your network and the Internet to see if it meets certain criteria. If it does, it is routed between the networks, otherwise it is stopped. A network firewall filters both inbound and outbound traffic. It can also manage public access to private networked resources such as host applications. It can be used to log all attempts to enter the private network and trigger alarms when hostile or unauthorized entry is attempted. Firewalls can filter packets based on their source, destination addresses and port numbers. This is known as address filtering. Firewalls can also filter specific types of network traffic. This is also known as protocol filtering because the decision to forward or reject traffic is dependant upon the protocol used, for example HTTP, ftp or telnet. Firewalls can also filter traffic by packet attribute or state.