A port number in a client request to a server is typically the destination port when it identifies the server application.

### Why this is right The server application is the destination of the client's request, so the port identifying that application is the destination port in the request. When the server replies, that service port is the source port. ### The concept A port is a field in a TCP or UDP transport header that helps a receiving device direct traffic to an application. The port's source or destination role depends on the direction of the packet. A client request travels toward the server, so the server's service port is the request's destination port. The reply travels from the server, so that same service port appears as the reply's source port. Port numbers help applications process traffic, but the number alone does not guarantee that the traffic is valid or that delivery succeeded. ### Terms - **TCP**: Transmission Control Protocol. A transport protocol that supports reliable delivery. - **UDP**: User Datagram Protocol. A transport protocol that provides best-effort delivery. - **Port number**: A transport-header value used to direct traffic to an application. - **Destination port**: The port associated with the receiving application for a packet. - **Source port**: The port associated with the sending application for a packet. - **Transport header**: The part of a packet that carries transport information, including port numbers. ### Remember Name the packet's direction first: the server's application port is a destination on the request and a source on the reply.