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Port Number

Each single network node can run multiple applications that use the same network interface, and therefore use the same IP address. For example, an RTOS application can run a TFTP server, an echo server and a Nabto client at the same time - all of which make use of the TCP/IP stack. Different applications running on the same network node, and therefore at the same IP address, are identified by their port number.

The source and destination address of each TCP or UDP packet is therefore a combination of an IP address and a port number. The IP address identifies the node on the network and the port number identifies the application within the node (see sockets).

In FreeRTOS-Plus-TCP the port number is unique across the system, so each port number can only be bound to a single socket across the entire system. This ensures that network code remains simple and communication is efficient and has a low memory footprint.

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