Binding

Each socket needs a unique address. As already stated, an address is the combination of an IP address and a port number.

When a socket is created it assumes the IP address of the network node that created it. If a socket has an IP address but not a port number it is said to be 'unbound'. An unbound socket cannot receive data because it does not have a complete address.

When a socket has both an IP address and a port number it is said to be 'bound to a port', or 'bound to an address'. A bound socket can receive data because it has a complete address.

The process of allocating a port number to a socket is called 'binding'.

The API function FreeRTOS_bind() is used to bind a FreeRTOS+TCP socket to a port number.

If ipconfigALLOW_SOCKET_SEND_WITHOUT_BIND is set to 0 in FreeRTOSIPConfig.h then FreeRTOS_bind() must be used to bind a socket to a port number before the socket can be used to either send or receive data. If ipconfigALLOW_SOCKET_SEND_WITHOUT_BIND is set to 1 in FreeRTOSIPConfig.h then an unbound socket will be automatically bound to a port number the first time it attempts to send data (for UDP sockets) or connect (for TCP sockets), but can still only receive data after it has been bound.

Useful links:

• TCP/IP Networking Basics
• FreeRTOS+TCP networking tutorial

Other networking basics pages:

• Ethernet Networking and Addressing
• MTU - Maximum Transmission Unit
• IP Networking
• IP Network Addressing
• ARP - Address Resolution Protocol
• Subnets / Netmasks
• Gateways and Routers
• UDP Networking
• TCP Networking
• MSS - Maximum Segment Size
• Port Numbers
• Sockets
• Binding
• Clients and Servers
• Static IP Addresses
• DHCP - Dynamic Host Control Protocol
• Name Resolution
• DNS - Domain Name Service
• LLMNR - Link-local Multicast Name Resolution
• NBNS - NetBIOS Name Service
• Bytes Order and Endian