Transport Level

Among other things, the transport level supplies a reliable communication channel. In addition, TCP contains mechanisms to handle congestion and control flow. The concept of port is introduced.The port is a 16-bit number (0…65535) that applications use to define the communication channel.Some ports can be reserved for certain applications assigned by the IANA (Internet Assigned Numbers Authority). Reserved ports are numbered 0 to 1023.The most used are:

• Port 80 – Default port for HTTP communication
• Port 20 / 21 – FTP service
• Port 23 – Telnet
• Port 502 – Modbus TCP

The following are added for Gefran systems:

• Port 5500 / 5800 / 5900 – VNC
• Port 5001…5010 – Java
• Port 500 – PLC