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binary numbers, the total number of ports is 2^16, or 65,536,
numbered from 0 through 65,535. A port is little more than an address
number that both ends of the communication link agree to use when
transferring data within the Transport layer. Ports allow a single IP
address to be able to support multiple simultaneous communications,
each using a different port number. The combination of an IP address
and a port number is known as a socket.
The first 1,024 of these ports (0–1,023) are called the well-known
ports or the service ports. This is because they have standardized
assignments as to the services they support. For example, port 80 is
the standard port for web (HTTP) traffic, port 23 is the standard port
for Telnet, and port 25 is the standard port for SMTP. These ports are
reserved for use exclusively by servers (in other words, they cannot be
used as the source port by a requesting client). You can find a list of
ports worth knowing for the exam in the section “Common Application
Layer Protocols” later in this chapter.
Ports 1,024 to 49151 are known as the registered software ports.
These are ports that have one or more networking software products
specifically registered with the International Assigned Numbers
Authority (IANA, www.iana.org) in order to provide a standardized
port-numbering system for clients attempting to connect to their
products.
Ports 49152 to 65535 are known as the random, dynamic, or
ephemeral ports because they are often used randomly and
temporarily by clients as a source port. These random ports are also
used by several networking services when negotiating a data transfer
pipeline between client and server outside the initial service or
registered ports, such as performed by common FTP.
Port Numbers
The IANA recommends that ports 49152 to 65535 be used as
dynamic and/or private ports. However, not all OSs abide by this.
A site that has a list of examples of the various ranges used by OSs
for random source ports is
