A system designed to block illegal access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both. All messages entering or leaving the Intranet pass through the firewall, which check each message and block those that do not meet the specified security criteria. This protects the internal network from intruders or hackers who might try to use the Internet to break into those systems.

There are several types of firewall techniques:

• Packet filter: Looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules. Packet filtering is adequately sufficient and transparent to users, but it is difficult to configure. It is also open to IP spoofing. To spoof an IP, a hacker must first use a variety of ways to find an IP address of a trusted port and then adjust the packet headers so that it looks like that the packets are coming from that port. Newer routers and firewall arrangements can offer protection against IP spoofing.

• Application gateway: Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can decrease a performance.

• Circuit-level gateway: Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.

• Proxy server: Intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses.

In practice, many firewalls use two or more of these techniques together.

A firewall is considered as a first line of defense in protecting private information. For greater security, data can be encrypted.

This will increase your computer’s security, but you also need to have a good ISP that has a good firewall in their system. You can find out more information about your ISP’s firewall by contacting your ISP’s technical service.

Some of the good firewalls that can be used on home computers as follows:
ZoneAlarm Free Firewall - As you can understand from its name, this one is an entry level firewall that you can use it for free
ZoneAlarm Pro - This is the paid version of the above firewall that additionally has identity theft and anti-spyware protection.
Online Armor Personal Firewall - This award winning firewall is available for Free. Paid version is also available with additional features.
Comodo Firewall - Free Firewall for personal use