What is Firewall Firewall is actually a system of security for the computers using the network connections either for the internet or for any other purpose. It prevents the unauthorized access from any sort of private computer to your own network. Firewalls can be installed in the computer in both ways either as hardware or software or sometimes used as a combination. It is required for the prevention of the trespassing of ones private networking connected to the internet, especially involving the use of the intranet. The working criteria of the firewall is such that every data transmission of the internet first passes through the firewall system and only those messages which pass a certain security criteria are allowed to get through; otherwise the other data is blocked, making it inaccessible for the unauthorized users. The firewall involves several different techniques one of them is the Packet filter. This is a simple technique of security for the firewall system. It keeps a check on the packets which enter and leave your computer network and then verifies them according to rules previously defined by your network user so as to allow or reject their passage through the network. The packet filtering is transparent to the network users, but its configuration is difficult and requires professional help. The other firewall techniques include the Application gateway, which is targeted to specific programs such as the FTP, followed by the Circuit-level gateway which is required for the TCP and the UDP connections. And lastly the Proxy servers, which is most commonly used these days for the internet security system. Firewall is actually considered to be the primary and the first line of defense for the security reasons.