In TCP or UDP communications, which fields specify the port numbers?

• A. Source and destination ports in the header
• B. IP addresses in the header
• C. Sequence numbers in the header
• D. Checksum fields in the header

Answer: (A)

In TCP and UDP, ports identify the sending and receiving applications at the ends of a connection. The transport-layer header includes two fields: the source port and the destination port. These specify which processes on each host are communicating and enable the network stack to demultiplex incoming data to the correct application. IP addresses are in the IP header, not the transport header, and are used to find the host itself. Sequence numbers are used by TCP to order data and manage reliability, not to indicate ports. Checksums verify integrity, not port information. So the fields that specify port numbers are the source and destination ports in the transport header.