防火墙工作原理简述（Brief introduction of firewall working principle）.doc

防火墙工作原理简述（Brief introduction of firewall working principle） How does a firewall work? All Internet communications are done through independent packet switching. Each packet is transmitted from the source host to the destination host. Packets are the basic unit of information transfer on the Internet. Although we often say connection between computers, the connection is actually composed of independent packets that are transmitted between two computers that are connected. In essence, they agree to connect to each other, and each send a response packet to the sender to let the sender know that the data is received. In order to reach the destination?? whether the two computer is through two steps or on different continents?? each Internet packet must contain a destination address and port number, and the source host IP address and port number, so that the recipient know who sent this package. That is, every packet that is sent on Internet must have both active and target addresses. An IP address always points to a separate machine on the Internet, while the port number is associated with a service or session on the machine. So, what does that mean? Check your firewall since the arrival of each computer data packets, then, before any software running on this machine on the package you see, firewall has completely veto anything, can ban your computer on receiving internet. A TCP/IP port is opened when the first request to establish the connection packet is answered by your computer. If the arriving packet is not accepted, the port will quickly disappear from the Internet and no one wants to connect with it. But the real strength of the firewall lies in choosing which packages to intercept and which packages should be released. Since the arrival of each packet contains the correct senders IP address (in order to send the recipient response packet, then some combination) source host IP address and port number and the target host IP address and port number based on firewall