Information Technology Reference
In-Depth Information
Similar issues arise for many applications, not just word proces
sors, and care must be taken in saving files to anticipate how they
might be used and who might use them on what machine(s).
When someone wants a file from a server, how does
the server know whether to use FTP, remote access,
or other programs?
Most computers send messages over an Ethernet network following
a standard approach called Transmission Control Protocol/Internet
Protocol (TCP/IP) . As part of the Internet Protocol, computers are re
quired to include information about which application should be used
for each message. This information is stored in the message's title, or IP
header. To be somewhat more precise, an IP header includes a port ad
dress (actually a 16bit number) that indicates the program that should
receive the included data. For example, port 21 is the identifier used to
indicate that a message contains FTP commands. Thus, when a com
puter receives a message specifying port 21, the machine knows to pass
the information along to the program handling FTP. Therefore, when
Karen posts her draft of the group paper on the common file server, her
computer sends the “upload” command in a message specifying port
21. The server then knows that the message will contain an FTP com
mand and presents that message to its FTP program. Similarly, port 20
indicates that the message contains file data related to an FTP com
mand. Thus, after an FTP user types the command “get finalpaper.rtf”,
the user's computer packs the command into a message that includes an
identifier indicating “port 21.” When the file server receives the message
from the network, it reads the “port 21” field, realizes the message has
to do with FTP, and sends the “get finalpaper.rtf” command to its FTP
program. When FTP on the server processes the command, it locates the
relevant file, divides the data into messages (called packets), adds the
designator “port 20,” and sends the data back to the client. When the
client receives a message indicating “port 20,” the client computer
knows the information received has to do with FTP file data, and the
data then are forwarded to the relevant FTP filehandling program.
The use of port numbers can work smoothly, provided all comput
ers agree on which numbers to use for which applications. For exam
ple, 21 indicates an FTP command, and 80 indicates access to the
World Wide Web. In fact, the nonprofit Internet Corporation for
Assigned Names and Numbers (ICANN) maintains a list of well
known port numbers, under contract with the United States govern
ment, the Internet Society, and other groups. Such assigned ports are
 
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