|  | 8.6 | TCP/IP Fundamentals
| 8.6.7 | The port numbers used (for example HTTP, FTP, SMTP) for a given service
Each TCP/IP protocol has at least one specifically designated port number for the flow of network traffic between client and server. Note that the use of the word "ports" here refers to structures in software, not hardware interfaces. Port numbers correspond to services to be provided for the upper layers. Well known port numbers are normally assigned by TCP/IP on the server prior to connections; however, these numbers can be changed. Because clients have the ability to dynamically log on to the server from any location, clients do not have assigned port numbers. For instance, if an individual (client) were to call a friend (server) from anywhere in the country, the friend would be able to pick up his or her telephone. Common port numbers include HTTP (port 80), FTP (port 21), and SMTP (port 25). -