User's Manual

Ports
User Guide 7
Although some services are essential, they can also be a security risk.
To send and receive data, you must “open a door” in your computer,
which makes your network vulnerable. One of the most common
ways networks are broken into is by intruders exploiting services.
Ports
On computers and other telecommunication devices, a port is a spe-
cific place for physically connecting another device, usually with a
socket and plug. A computer usually has one or more serial ports
and one parallel port. The serial port supports sequential, one bit-
at-a-time transmission to devices such as scanners, and the parallel
port supports multiple-bit-at-a-time transmission to devices such as
printers.
Computers also have ports that are not physical locations. These
ports are “logical connection places” for programs or applications on
a computer in a network. Some applications, such as HTTP, have
ports with preassigned numbers. These are known as “well-known
ports.” Other application processes are assigned port numbers
dynamically for each connection. When a service is initially started,
it is said to “bind” to its designated port number.
Every Internet service using TCP is identified by a unique port num-
ber. When a client initiates a connection to a server, it chooses to
connect to, say, port 25 on the remote machine. Port 25 is assigned
to the SMTP protocol which is the service of delivering electronic
mail.
Most services are assigned a port number in the range from 0 to
1024, but the valid port numbers range from 0 to 65535.