A firewall is a piece of hardware or software (or a combination of both) designed to be a first line of defense against an unauthorized user accessing a private network. More commonly, firewalls are used in connection with Internet sites. Think of firewalls as gatekeepers. All messages coming in or going out are carefully screened to meet certain select security criteria.
Some common firewall techniques include:
Packet Filter: This technique looks at each packet entering or leaving a network, accepting or rejecting it based on established rules. Packet filering is fairly effective and transparent to users, however it is often difficult to configure. It is also vulnurable to Denial of Service attacks.
Application Gateway: This method is used for specific applications, such as FTP and Telenet. This is also effective, but can result in performance loss.
Circuit-Level Gateway: This is also used for specific applications, such as TCP or UDP. Once a connection has been established, packets can flow between the hosts without further checking.
Proxy Server: This method intercepts all messages entering and leaving a network. The proxy server essentially hides the network's true address.
Many firewalls employ two or more of the above techniques. For additional security, a company may also encrypt its internal data so that only authorized users have the correct key.
Short for User Datagram Protocol, a connectionless protocol that, like TCP, runs on top of IP networks. Unlike TCP/IP, UDP/IP provides very few error recovery services, offering instead a direct way to send and receive datagrams over an IP network. It's used primarily for broadcasting messages over a network.