Multiplexing and Demultiplexing using port numbers
Transport Layer protocols (TCP and UDP) are responsible for supporting multiple network applications at the same instance and these applications can send and receive network data simultaneously. Transport Layer Protocols are capable of doing this by making use of application level addressing, known as port numbers. The data from different applications operating on a network device are multiplexed at the sending device using port numbers and demultiplexed at the receiving device, again using port numbers.

TCP UDP Port Numbers
The two 16 bit fields in the TCP Header, Source port and Destination port identifies the port number which the application is listening at the sending device and receiving device. Since port number is a 16 bit number, the maximum possible value is 65535 ((2^16)-1).

The port numbers are divided into three ranges.

• The Well Known Ports are those in the range 0 - 1023. The Well Known Ports are assigned by the IANA for major protocols.
• The Registered Ports are those in the range 1024 - 49151.
• The Private Ports are those in the range 49152 - 65535.