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Description TCP and UDP are both transport protocols above the IP layer, which are interfaces between IP and upper-layer processes. TCP and UDP protocol port numbers are designed to distinguish multiple applications running on a single device with one IP address from one another.

Since many network applications may be running on the same machine, computers need something to make sure the correct software application on the destination computer gets the data packets from the source machine, and to make sure replies get routed to the correct application on the source computer. This is accomplished through the use of the TCP or UDP "port numbers". In the TCP and UDP header, there are "Source Port" and "Destination Port" fields which are used to indicate the message sending process and receiving process identities defined. The combination of the IP address and the port number is called "socket".

There are three port ranges defined by IETF IANA: the Well Known Ports, the Registered Ports, and the Dynamic and/or Private Ports.

• The Well Known Ports are in the range of 0 to 1023, which are assigned by the IANA. In most cases, they can only be used by system (or root) processes or by programs executed by privileged users.
• The Registered Ports are in the range of 1024 to 49151, which are not controlled by IANA. They are commonly used by ordinary user processes or programs executed by ordinary users.
• The Dynamic and/or Private Ports are in the range of 49152 to 65535, which are typically used as source port by a TCP or UDP client, to communicate with a remote TCP or UDP server, using a well-known port as destination port.

Partial TCP UDP Port Numbers (Well-Known Ports)

Reference Links

http://www.iana.org/assignments/port-numbers
TCP and UDP port numbers