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Unformatted text preview: /D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/introduc.htm (12 of 20) [12/09/2001 14.46.31] Chapter 1. Introduction 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
says it uses TCP port 23
sun % grep domain /etc/services
says it uses UDP port 53
and TCP port 53
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. 1.10 Standardization Process
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