A.4.3 IP Sockets and Ports

In the TCP/IP world, applications use what are commonly referred to as "well-known ports" to communicate with each other. Client applications assume that the corresponding server application is listening on the well-known port associated with that application.

For example, the well-known port number for HTTP, the protocol used to transfer HTML pages across the World Wide Web, is TCP port 80. By default, a web browser will attempt to open a connection on the destination host's TCP port 80, unless another port number is specified in the URL (such as 8000 or 8080). Figure A.15 below illustrates this concept:

Figure A.15 By using well-known ports, client applications such as web browsers can connect directly to the web server running on a remote system.

Server applications typically load during system startup, and actively "listen" for incoming connections on their well-known port. Client applications will then attempt to connect to the server, and a TCP or UDP exchange will then take place. When the session is complete, the client will typically be the one to terminate the connection.

For more information on this subject, refer to section B.5.1 Port Numbers and Sockets. For more information on the IP protocol, refer to section A.4.1 Internet Protocol (IP) and Address Resolution Protocol (ARP). For more information on IP routing, refer to section A.4.4 Routing Information Protocol (RIP) and Open Shortest Path First (OSPF). For more information on name resolution services, refer to section A.4.5 HOSTS Files and the Domain Name Service (DNS). For more information on TCP/IP's mail protocols, refer to Appendix D: SMTP and POP3 Mail.

Send comments to docs@unoverica.com