Whether you are trying to protect a home computer or corporate intranet a firewall is one of the primary methods of helping to secure your resources. A firewall is a program or hardware device used to limit access between a public network and a computer or private network. The firewall can specify what programs, ports, or machines can actually access resources that are hidden behind its barrier.
Firewalls control traffic coming in and out of networks in three ways: Packet filtering, proxy service, and stateful inspection. Any packet of information that meets the criteria of packet filters is flagged and prevented from accessing any internal resources. For example if port 21, which is the default port for the file transfer protocol on a windows machine, is not specified in the firewall settings to be allowed to pass through then the ftp request is blocked. The external user or machine sending the ftp request will receive a connection error on their end. The proxy service acts as a router and sends information to the appropriate system. According to Howstuffworks.com a stateful inspection is, “a newer method that doesn't examine the contents of each packet but instead compares certain key parts of the packet to a database of trusted information. Information traveling from inside the firewall to the outside is monitored for specific defining characteristics, then incoming information is compared to these characteristics. If the comparison yields a reasonable match, the information is allowed through. Otherwise it is discarded.