Information Technology Reference
In-Depth Information
numbered between 0 and 1023. Because applications change over time,
the list of these port numbers also changes. A current list of assigned
numbers is now maintained in an online database; although responsi
bility for such matters continues to evolve, if you're interested, you
might search the Web at
http://www.icann.org
for various details. You
could also search under the name of ICANN's predecessor, the Internet
Assigned Numbers Authority (IANA). Table 9.1 shows assigned port
numbers for some common applications.
Although numbers above 1023 are not formally assigned, the
ICANN does maintain a registry of numbers above 1023 that are
used for various applications. When new applications are created,
the writers can officially register their port numbers so that the com
puter community will know which port numbers they use. (If you
want to claim a port number for your application, you should con
tact ICANN about their procedures.) Nonregistered applications
may use registered port numbers for communication, as long as the
client and server applications agree on which port to use. To avoid
interfering with other applications, however, those who are writing
new applications are encouraged to review the current registry be
fore deciding what numbers their applications might use and choose
unique numbers for their applications. For whichever ports the writ
ers use, in order to conform to TCP/IP, the port numbers will be
contained in the IP header.
As you might expect, transmitting materials from one computer to
another requires many details to fit together smoothly. To successfully
coordinate all the activity that must take place for communication,
Table 9.1 Some Common Port Numbers
Port Number
(Decimal)
Application Description
20
FTP data
21
FTP control
23
Telnet (a simple approach for logging into
computers at a distance)
25
Email (Simple Mail Transfer Protocol)
80
HTTP, used for the World Wide Web
