In an IP datagram, which of the following fields identifies the receiving application?

An IP datagram encapsulates a transport layer (Layer 4) segment, such as TCP or UDP. This transport layer header contains source and destination port numbers. The destination host's operating system uses the destination port number to identify the specific application process that should receive the incoming data. This process is known as demultiplexing. Each application that wishes to receive network data listens on a unique port number, allowing the OS to correctly forward data from the network stack to the intended application.

A. The protocol field of the transport layer header. Transport layer headers (TCP/UDP) do not have a "protocol" field; they use port numbers to identify endpoints. The protocol is identified at the network layer.
C. The protocol field of the network layer header. This field identifies the transport layer protocol being used (e.g., TCP=6, UDP=17), not the specific end-user application.
D. The port field of the network layer address. Network layer addresses (IP addresses) do not have port fields. Ports are a construct of the transport layer (Layer 4), not the network layer (Layer 3).

1. Nokia Scalable IP Networks Self-Study Guide (4A0-113), Module 2: The TCP/IP Model, Section: "Transport Layer". This section details that the transport layer uses port numbers to multiplex and demultiplex data streams between different applications on a host. The destination port number specifically identifies the receiving application.

2. Postel, J. (1981). RFC 793: Transmission Control Protocol. Internet Engineering Task Force (IETF). Section 3.1, "Header Format". This document defines the TCP header, which includes the "Destination Port" field used to identify the receiving process on the destination host. Available at: https://doi.org/10.17487/RFC0793

3. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. Chapter 3, Section 3.2, "Multiplexing and Demultiplexing". This university-level textbook explicitly states, "The job of delivering the data in a transport-layer segment to the correct socket is called demultiplexing... the host uses the destination port number to direct the segment to the appropriate socket." (p. 207).