Once data is routed through the network and delivered to a specific host, it must be delivered to the correct user or process. As the data moves up or down the layers of TCP/IP, a mechanism is needed to deliver data to the correct protocols in each layer. The system must be able to combine data from many applications into a few transport protocols, and from the transport protocols into the Internet Protocol. Combining many sources of data into a single data stream is called multiplexing. Data arriving from the network must be demultiplexed: divided for delivery to multiple processes. To accomplish this, IP uses protocol numbers to identify transport protocols, and the transport protocols use port numbers to identify applications.
Some protocol and port numbers are reserved to identify well-known services. Well-known services are standard network protocols, such as FTP and TELNET, that are commonly used throughout the network. The protocol numbers and port numbers allocated to well-known services are documented in the Assigned Numbers RFC. Windows NT systems define protocol and port numbers in two simple text files.
The protocol number is a single byte in the third word of the datagram header. The value identifies the protocol in the layer above IP to which the data should be passed.
On an NT system, the protocol numbers are defined in the protocol file. This file is a simple table containing the protocol name and the protocol number ...