Section 6 describes the different port number ranges. It is important to note that IANA applies slightly different procedures when managing the different port ranges of the service name and port number registry:
o Ports in the Dynamic Ports range (49152-65535) have been specifically set aside for local and dynamic use and cannot be assigned through IANA. Application software may simply use any dynamic port that is available on the local host, without any sort of assignment. On the other hand, application software MUST NOT assume that a specific port number in the Dynamic Ports range will always be available for communication at all times, and a port number in that range hence MUST NOT be used as a service identifier.
o Ports in the User Ports range (1024-49151) are available for assignment through IANA, and MAY be used as service identifiers upon successful assignment. Because assigning a port number for a specific application consumes a fraction of the shared resource that is the port number registry, IANA will require the requester to document the intended use of the port number. For most IETF protocols, ports in the User Ports range will be assigned under the "IETF Review" or "IESG Approval" procedures [RFC5226] and no further documentation is required. Where these procedures do not apply, then the requester must input the documentation to the "Expert Review" procedure [RFC5226], by which IANA will have a technical expert review the request to determine whether to grant the assignment. Regardless of the path ("IETF Review", "IESG Approval", or "Expert Review"), the submitted documentation is expected to be the same, as described in this section, and MUST explain why using a port number in the Dynamic Ports range is unsuitable for the given application. Further, IANA MAY utilize the "Expert Review" process informally to inform their position in participating in "IETF Review" and "IESG Approval".
o Ports in the System Ports range (0-1023) are also available for assignment through IANA. Because the System Ports range is both the smallest and the most densely assigned, the requirements for new assignments are more strict than those for the User Ports range, and will only be granted under the "IETF Review" or "IESG Approval" procedures [RFC5226]. A request for a System Port number MUST document *both* why using a port number from the Dynamic Ports range is unsuitable *and* why using a port number from the User Ports range is unsuitable for that application.
