A firewall is a device or group of devices that controls access between networks. A firewall generally consists of filters and gateway(s), varying from firewall to firewall. It is a security gateway that controls access between the public Internet and an intranet (a private internal network) and is a secure computer system placed between a trusted network and an untrusted internet. A firewall is an agent which screens network traffic in some way, blocking traffic it believes to be inappropriate, dangerous, or both. The security concerns that inevitably arise between the sometimes hostile Internet and secure intranets are often dealt with by inserting one or more firewalls in the path connecting the Internet and the internal network. In reality, Internet access provides benefits to individual users, government agencies and most organisations. But this access often creates a threat as a security flaw. The protective device that has been widely accepted is the firewall. When inserted between the private intranet and the public Internet it establishes a controlled link and erects an outer security wall or perimeter. The aim of this wall is to protect the intranet from Internet-based attacks and to provide a choke point where security can be imposed. Firewalls act as an intermediate server in handling SMTP and HTTP connections in either direction. Firewalls also require the use of an access negotiation and encapsulation protocol such as SOCKS to gain access to the Internet, the intranet, or both. Many firewalls support tri-homing, allowing use of a DMZ network. It is possible for a firewall to accommodate more than three interfaces, each attached to a different network segment. Firewalls can be classified into three main categories: packet filters, circuit-level gate ways and application-level gateways.