The Main function of a firewall is to protect the network from an untrusted network, by filtering the packets passing throw it, the process of filtering those packets is done across a secure policy. A firewall in a computer network performs a role that is very similar to that of a firewall in a building. Just as a firewall made out of concrete protects one part of a building, a firewall in a network ensures that if something bad happens on one side of the firewall, computers on the other side won’t be affected. Unlike a building firewall, which protects against a very specific threat (fire), a network firewall has to protect against many different kinds of threats like Viruses, Worms, DoS [1, 11-16]

II. TERMS AND VOCABULARY

In order to understand how firewall works, we have to know a little about networks, here is some important terms: Firewall: S. Cobb [2] define a firewall as a collection of components or a system that is placed between two networks and possesses the following properties: • All traffic from inside to outside, and vice-versa, must pass through it. • Only authorized traffic, as defined by the local security policy, is allowed to pass through it. • The firewall itself is immune to penetration. Packet: A packet is the unit of data that is routed between an origin and a destination on the Internet or any other network. When any file (e-mail message, HTML file, Graphics Interchange Format file, Uniform Resource Locator request, and so forth) is sent from one place to another on the Internet, the Transmission Control Protocol (TCP) layer of TCP/IP divides the file into “chunks” of an efficient size for routing. Each of these packets is separately numbered and includes the Internet address of the destination. The individual packets for a given file may travel different routes through the Internet (or the network). When they have all arrived, they are reassembled into the original file (by the TCP layer at the receiving end) [11-16]. TCP/IP: In order to understand the TCP/IP we quote from this article[4]: TCP is a protocol that runs on top of IP. IP takes care of delivering packets. For the remainder of this article, it is relevant that each IP packet contains two addresses: the address of the source of the packet and the address of the destination of the packet. TCP adds a reliable, connection oriented service to IP. It makes sure that there is a reliable data stream between source and destination. TCP avoids duplication of transmitted data and avoids delivering data out of order. In order to be able to setup multiple connections between two hosts, TCP adds so-called ports to IP addresses. These ports identify the connection endpoints on the source and destination. The combination of source port, source address, destination port and destination address are unique for every TCP connection. TCP uses a three-way handshake to setup a connection.