Port 723 occupies space in the well-known ports range (0-1023) but appears to have no official service assigned by IANA. This makes it unusual—most ports in this range have been formally allocated to specific protocols and services.
What the Well-Known Range Means
Ports 0-1023 are called System Ports or well-known ports. They're assigned by IANA using strict procedures requiring IETF Review or IESG Approval.1 These ports are reserved for standardized services that need consistent port numbers across all systems—HTTP on 80, HTTPS on 443, SSH on 22.
Because of this controlled assignment process, most ports in this range have official designations. Unassigned ports like 723 are relatively rare.
Historical and Unofficial Use
Some sources reference port 723 as being used by Mac OS X RPC-based services, particularly NetInfo.2 NetInfo was Apple's distributed database system for managing administrative data—user accounts, network configuration, printers, and other resources—used in NeXTSTEP, OpenStep, and early versions of Mac OS X.
NetInfo operated through RPC (Remote Procedure Call) mechanisms and used various ports in the 600-1023 range. Apple deprecated NetInfo in OS X 10.5 Leopard (2007), replacing it with Directory Services and later Open Directory.3
Whether port 723 was ever formally part of NetInfo's operation or just appeared in unofficial documentation is unclear. No RFC or IANA record confirms an official assignment.
Why Unassigned Ports Matter
The existence of unassigned ports—even in the controlled well-known range—serves several purposes:
Room for future standards — New protocols may need official port assignments. Having gaps in the registry allows for future allocation without conflicts.
Avoiding premature standardization — Not every service that once existed deserved a permanent port assignment. NetInfo was Apple-specific and eventually abandoned. Leaving 723 unassigned avoided locking in a proprietary service.
Local and experimental use — Unassigned ports can be used for internal applications, testing, or temporary services without conflicting with standardized protocols.
Checking What's Listening
Even though port 723 has no official assignment, something could be listening on it on your system. Here's how to check:
On Linux or macOS:
On Windows:
If nothing returns, the port is closed. If you see a process, investigate what's running—it could be legitimate software, a legacy service, or something unexpected.
Related Ports
Other ports in the well-known range that are either unassigned or have historical/specialized uses:
- Port 720 — Unassigned
- Port 721 — Unassigned
- Port 722 — Unassigned
- Port 724 — Unassigned
The cluster of unassigned ports around 723 suggests this section of the registry was never heavily utilized for standardized services.
Security Considerations
Unassigned ports aren't inherently more or less secure than assigned ones. What matters is whether something is listening and how it's configured.
If you find an unexpected service on port 723:
- Identify what process owns it
- Determine if it's legitimate software or an unauthorized service
- Check firewall rules to ensure it's not exposed to the Internet unnecessarily
- If it's not needed, stop the service and close the port
Frequently Asked Questions About Port 723
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