1.9 Port Numbers

We said that TCP and UDP identify applications using 16-bit port numbers. How are these port numbers chosen ?

Servers are normally known by their well-known port number. For example, every TCP/IP implementation that provides an FTP server provides that service on TCP port 21. Every Telnet server is on TCP port 23. Every implementation of TFTP (the Trivial File Transfer Protocol) is on UDP port 69. Those services that can be provided by any implementation of TCP/IP have well-known port numbers between 1 and 1023. The well-known ports are managed by the Internet Assigned Numbers Authority (IANA).

Until 1992 the well-known ports were between 1 and 255. Ports between 256 and 1023 were normally used by Unix systems for Unix-specific services ” that is, services found on a Unix system, but probably not found on other operating systems. The IANA now manages the ports between 1 and 1023.

An example of the difference between an Internet-wide service and a Unix-specific service is the difference between Telnet and Rlogin. Both allow us to login across a network to another host. Telnet is a TCP/IP standard with a well-known port number of 23 and can be implemented on almost any operating system. Rlogin, on the other hand, was originally designed for Unix systems (although many non-Unix systems now provide it also) so its well-known port was chosen in the early 1980s as 513.

A client usually doesn't care what port number it uses on its end. All it needs to be certain of is that whatever port number it uses be unique on its host. Client port numbers are called ephemeral ports (i.e., short lived). This is because a client typically exists only as long as the user running the client needs its service, while servers typically run as long as the host is up.

Most TCP/IP implementations allocate ephemeral port numbers between 1024 and 5000. The port numbers above 5000 are intended for other servers (those that aren't well known across the Internet). We'll see many examples of how ephemeral ports are allocated in the examples throughout the text.

Solaris 2.2 is a notable exception. By default the ephemeral ports for TCP and UDP start at 32768. Section E.4 details the configuration options that can be modified by the system administrator to change these defaults.

The well-known port numbers are contained in the file /etc/services on most Unix systems. To find the port numbers for the Telnet server and the Domain Name System, we can execute

 sun  %  grep telnet /etc/services  telnet     23/tcp  says it uses TCP port 23  sun  %  grep domain /etc/services  domain     53/udp  says it uses UDP port 53  domain     53/tcp  and TCP port 53

Reserved Ports

Unix systems have the concept of reserved ports. Only a process with superuser privileges can assign itself a reserved port.

These port numbers are in the range of 1 to 1023, and are used by some applications (notably Rlogin, Section 26.2), as part of the authentication between the client and server.