Protocol Field (Network Layer) vs Port Numbers (Transport Layer)

Common purpose

Both fields are demultiplexing identifiers. Many protocols / many applications share the same physical network and the same IP address, so at the receiver something must say which upper-layer entity should get the payload. Both fields answer exactly that question — they let several users of a channel coexist (multiplexing at the sender, demultiplexing at the receiver).

Comparison

Feature	Protocol field (IP header)	Port numbers (TCP/UDP header)
Layer	Network layer (Layer 3)	Transport layer (Layer 4)
Size	8 bits	16 bits each
Number of fields	One	Two (source + destination)
Identifies	The next-layer protocol to which the payload is handed (TCP = 6, UDP = 17, ICMP = 1, IPv6 = 41)	The sending/receiving process (application socket)
Value assigned by	The sending host's IP layer	The OS/application — well-known ports 0–1023, ephemeral 49152–65535
Demultiplexing done by	Receiver's IP layer → protocol module	Receiver's transport layer → process
Namespace	Small, closed set of registered protocols	Very large, dynamic set of concurrent applications

Why two port fields but only one protocol field?

A received IP datagram contains the payload of exactly one protocol — IP delivers it to one and only one next layer — so a single "next protocol" field completely identifies the destination; there is no "sender protocol" to represent, since IP itself is always the sender at that layer. A transport segment, however, is part of a two-way conversation: the receiver must know which remote process it came from (so it can reply) and which local process it is for. Without a source port the reply could not be routed back to the right socket at the original host — hence, exactly as with source and destination IP addresses, two port fields (source and destination) are required.

Why is the protocol field only half the size of a port number?

• The protocol field is 8 bits = 256 values, which is ample because only a few dozen protocols are registered at the network layer; it merely has to point to a subsystem (TCP, UDP, ICMP, …) inside the host — a small, essentially fixed, closed set.
• A port number is 16 bits = 65 536 values because it must name individual processes/sockets on one machine: every concurrent web, mail, DNS, SSH, streaming … connection needs its own port, and both well-known and dynamically assigned (ephemeral) ports share that range. Two 16-bit fields also yield 2³² distinct (source, destination) socket pairs per IP address pair, which is necessary for many simultaneous connections between the same two hosts.
• So the difference in size simply reflects the difference in the size of the two namespaces: protocol identification is coarse-grained, process identification is fine-grained.