Originally, port numbers were used by the Network Control Program (NCP) in the ARPANET for which two ports were required for half-duplex transmission.Later, the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) needed only one port for full-duplex, bidirectional traffic.They usually use port numbers that match the services of the corresponding TCP or UDP implementation, if they exist.The Internet Assigned Numbers Authority (IANA) is responsible for maintaining the official assignments of port numbers for specific uses.The even-numbered ports were not used, and this resulted in some even numbers in the well-known port number range being unassigned.
Similarly many of the official assignments refer to protocols that were never or are no longer in common use.
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It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on (Africa), ftp.(Europe), au (Pacific Rim), ds.(US East Coast), or edu (US West Coast).
Historically, other directory services preceding DNS were not scalable to large or global directories as they were originally based on text files, prominently the HOSTS. The Internet maintains two principal namespaces, the domain name hierarchy The Domain Name System maintains the domain name hierarchy and provides translation services between it and the address spaces.
This is a list of notable port numbers used by protocols of the transport layer of the Internet protocol suite for the establishment of host-to-host connectivity.