1.12 Standard, Simple Services

There are a few standard, simple services that almost every implementation provides. We'll use some of these servers throughout the text, usually with the Telnet client. Figure 1.9 describes these services. We can see from this figure that when the same service is provided using both TCP and UDP, both port numbers are normally chosen to be the same.

Figure 1.9. Standard, simple services provided by most implementations .

If we examine the port numbers for these standard services and other standard TCP/IP services (Telnet, FTP, SMTP, etc.), most are odd numbers. This is historical as these port numbers are derived from the NCP port numbers. (NCP, the Network Control Protocol, preceded TCP as a transport layer protocol for the ARPANET.) NCP was simplex, not full-duplex , so each application required two connections, and an even-odd pair of port numbers was reserved for each application. When TCP and UDP became the standard transport layers , only a single port number was needed per application, so the odd port numbers from NCP were used.