Final answer:
The operating system uses the destination IP address, destination port number, source IP address, and source port number to direct a UDP segment to the appropriate socket.
Step-by-step explanation:
When a UDP segment arrives at a host, the operating system directs the segment to the appropriate socket using a combination of the destination IP address, the destination port number, the source IP address, and the source port number. This combination of information forms an identifier for the network communication, known as the socket. The destination port number is particularly important, as it signifies the specific application or process that should receive the UDP segment. Unlike TCP, UDP does not establish a connection, and therefore there's no connection state to help direct the segment. Instead, the OS relies purely on the above-mentioned identifiers to deliver the segment properly.