A firewall is either hardware, software, or both that limits outside attacks to a computer or a network.

Most commonly, firewalls are used on computers that have access to the internet, especially high-bandwidth connections. If a network, either home or corporate, does not have a firewall, practically anyone that has basic intrusion knowledge on the internet can get onto the LAN and retrieve any information on any of the computers connected to it, or can even spread viruses over the network.

The most common types of firewalls are:

Network-level firewall – Also known as packet filter. This type of firewall examines all inbound traffic at the network protocol level and determines which packets are safe.

Application-level firewall – This type of firewall examines all traffic at the application level, which can be FTP, e-mail or Telnet and determines security based on its configuration. Application-level firewalls can also readdress outgoing traffic so that it can appear to have originated from the firewall itself instead of the host computer.

Traditional firewalls can be implemented by both hosts and routers. At a router, it can control traffic examining each packet and allowing or blocking the packet, depending on port numbers that issued them. This is known as packet filtering. A firewall implemented on a host, on the other hand, works at the application level, providing much more security, due to the fact that it can control traffic through a more detailed examination based on protocols. The process through which packet traffic is examined and forwarded is known as proxy.

Although technologies based on proxies may provide lower internet access performance and the need for a dedicated proxy for each type of connectivity, it provides a much higher level of security as it does not allow any form of direct packet transfer between hosts and the internet. So, whether dealing with a single computer, a home LAN, or corporate network, firewalls are a necessity to assure safer internet access.

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