Microsoft say this about Winsock error 10048 - WSAEADDRINUSE:

Typically, only one usage of each socket address (protocol/IP address/port) is permitted. This error occurs if an application attempts to bind a socket to an IP address/port that has already been used for an existing socket, or a socket that was not closed properly, or one that is still in the process of closing. For server applications that need to bind multiple sockets to the same port number, consider using setsockopt (SO_REUSEADDR). Client applications usually need not call bind at all— connect chooses an unused port automatically. When bind is called with a wildcard address (involving ADDR_ANY), a WSAEADDRINUSE error could be delayed until the specific address is committed. This could happen with a call to another function later, including connect, listen, WSAConnect, or WSAJoinLeaf.

Full details of Winsock TCP error 10048 - WSAEADDRINUSE:

Berkeley description: Only one usage of each address is normally permitted.

WinSock description: Same as Berkeley. The "address" they refer to, typically refers to the local "socket name", which is made up of the 3-tuple: protocol, port-number and IP address.