Initially, there were not many threats to network security. With the Internet becoming more popular, more companies are doing business over the web. In addition, a number of attempts to illegally infiltrate networks have increased. As a result, the need to secure networks has grown. Firewalls were implemented on networks to prevent unauthorized access from external sources. Initially firewalls were complex & unreliable hardware devices. Most of the firewalls were routers that separate a private network from other networks. Over the years, firewalls have developed into highly reliable solutions. They protect networks from unauthorized access & play an important part in enforcing the security policy for a company. The firewall examines each packet of data that passes through it & allows the authorized data.