Firewall Primer

ports

When two networked computers communicate with one another (i.e., send data to each other in a mutually agreed upon manner) they do so using one of several different protocols . A protocol, much like that in human communication, is comprised of a precise and specific definition of how communication should start, continue and end. Network protocols include TCP/IP (Transmission Control Protocol/Internet Protocol), UDP (User Datagram Protocol) and ICMP (Internet Control Message Protocol).

Each protocol is designed for applications with specific requirements. For example, when two computers communicate using UDP, the computer that receives the data does not acknowledge receipt, whereas when TCP/IP is used the sender can ascertain whether the transmission has been received. This makes UDP more suitable for broadcast-type communication where less overhead is of benefit. On the other hand, TCP was designed to be robust and to maximize the likelihood of successful transmission.

Communication takes place between a port on one computer and another port on another computer. There are 65,536 available ports. Conventionally, the full range of ports is divided functionally into three groups which will be described shortly. A socket is a combination of a particular IP address and a port. For example, I can create a socket on my computer at a particular port (e.g., comprised of my IP address and port ssh/22) and wait for you to create your socket to connect to my socket. This is how communication starts: using a client socket on your computer you connect to my server socket.

A port is numbered and, if associated with an application agreed upon by IANA or other conventions, associated with an application protocol . An application protocol is a well-defined way for a server and client components of an application to communicate towards a mutual goal. For example, the communication between a mail server and a mail client is very different than the communication between a web server and a web browser. Mail clients need to know how to read a message, delete a message, download more mail headers, etc and communicate using the POP or IMAP protocols. Web clients need to know how to fetch web pages, process web page contents, upload form data, etc, and speak the HTTP protocol. Typically a given client application is designed to communicate with a particular class of server applictions using a single protocol.