What does a "write UDP unknown error code 10051" typically indicate?

Error code 10051, when encountered during a UDP write operation, generally signifies a "No connection could be made because the target machine actively refused it." While UDP is connectionless, this often means there's no application listening on the specified port at the destination IP address, or a firewall is actively blocking the UDP traffic. It doesn't necessarily mean a connection *was* refused, but rather that no process is accepting packets on that port.