Specifications

1 - 9
1.3.3 TCP and UDP
TCP and UDP are protocols which are processed by the devices and PCs on the both
ends of communication (Transport layer).
(1) Port Number
The actual communication is performed between the application programs
operated on devices and PCs.
TCP and UDP identify which application programs are communicating with each
other using the port number.
If the IP address is regarded as "address", the port number corresponds to “the
floor of a building".
Application program A
Application program B
Application program C
Application program D
Application program E
Address XX (IP address)
3rd floor of building
(Port No.)
IP network
2nd floor of building (Port No.)
Address YY (IP address)
In reality, combinations of the following five items identifiy individual
communication.
• Destination IP address
• Source IP address
• Destination port number
• Source port number
• Protocol number (TCP=6H, UDP=17H)