273 Unassigned - System Port

273 is a System Port. Transport Protocol , 273 Service Name , Service Name Description: Unassigned.

Port	Protocol	Service Name	Description	Source
273			Unassigned	IANA

Port	Service Name	Description
1587	pra-elmd	pra_elmd IANA assigned this well-formed service name as a replacement for "pra_elmd".
2154	stdptc	Standard Protocol
2691	itinternet	ITInternet ISM Server
3553	rbr-discovery	Red Box Recorder ADP
5012	nsp	NetOnTap Service
5289		Unassigned
7422		Unassigned
7457		Unassigned
8003	mcreport	Mulberry Connect Reporting Service
8210		Unassigned

A port is an endpoint of communication in an operating system. While the term is also used for hardware devices, in software it is a logical construct that identifies a specific process or a type of service.

Port numbers are assigned in various ways, based on three ranges: System Ports (0-1023), User Ports (1024-49151), and the Dynamic and/or Private Ports (49152-65535);

The Well Known Ports are assigned by the IANA and on most systems can only be used by system (or root) processes or by programs executed by privileged users. Ports are used in the TCP [RFC793] to name the ends of logical connections which carry long term conversations. For the purpose of providing services to unknown callers, a service contact port is defined. This list specifies the port used by the server process as its contact port. The contact port is sometimes called the "well-known port". To the extent possible, these same port assignments are used with the UDP [RFC768]. The range for well-known ports managed by the IANA is 0-1023.
DYNAMIC AND/OR PRIVATE PORTS Registered Ports are those from 1024 through 49151.
The Dynamic and/or Private Ports are those from 49152 through 65535