Port 273: Unassigned — A Blank Page in the Well-Known Range

Port 273 is officially unassigned. According to the IANA Service Name and Transport Protocol Port Number Registry, ports 272-279 are classified as "Unassigned."¹ There is no service here, no protocol, no story. Just an empty slot in the registry.

What Does "Unassigned" Mean?

Port 273 falls within the System Ports range (also called Well-Known Ports), which spans 0-1023. These ports are managed by IANA and are typically assigned through formal IETF review processes.² When a port in this range is marked "Unassigned," it means:

• No official service has been assigned to this port number
• The port is available for assignment if someone requests it through proper channels
• IANA has not reserved it for special purposes

This is distinct from "Reserved" ports, which IANA holds back for specific reasons.

The Well-Known Ports Range

The well-known ports range was created when the Internet was young. Ports 0-255 were the original assigned range, expanded later to 0-1023 as the Internet grew.³ Most of these ports have stories—SSH on 22, SMTP on 25, HTTP on 80, HTTPS on 443. Someone needed to solve a problem, wrote an RFC, and claimed a number.

Port 273 never got claimed. Neither did 272, 274, 275, 276, 277, 278, or 279. They sit together as a small block of unused space in a range where almost every other number carries decades of history.¹

Why Unassigned Ports Matter

The existence of unassigned ports in the well-known range is actually important:

Room for Growth: Even in 2026, new protocols occasionally need well-known port assignments. Having unassigned ports available means future protocols can claim official space.

Clean Namespace: IANA doesn't assign ports speculatively. If a port is unassigned, it genuinely has no official use. This clarity matters for network administrators and security tools.

No Unofficial Use: Unlike some unassigned ports in higher ranges that get claimed by applications anyway, port 273 has no known unofficial uses, no trojan associations, and no common applications listening here.¹

How to Check What's on Port 273

If you want to verify whether anything is listening on port 273 on your system:

On Linux/Unix:

# Using netstat
netstat -tulpn | grep ':273'

# Using lsof
sudo lsof -i:273

# Using ss (on newer systems)
ss -tulpn | grep ':273'

On Windows:

netstat -ano | findstr :273

If the output is empty, nothing is listening. Which is exactly what you should expect—because nothing official uses this port.⁴

The Silence

There is something genuinely strange about port 273. In the well-known range where almost every number tells a story about the Internet's evolution, this port and its neighbors (272-279) are blank pages. No RFC defined them. No protocol claimed them. No trojan even bothered squatting here.

They're just... unassigned. Waiting. Available.

Maybe someday someone will need port 273 for something important. Until then, it sits in the registry like a blank line in a poem—present, counted, but carrying no meaning yet.

Related Ports

• Port 271 — PT-TLS (Posture Transport Protocol over TLS), the assigned port immediately before the unassigned block¹
• Port 280 — http-mgmt (HTTP Management), the first assigned port after the unassigned block
• Ports 272-279 — The complete block of unassigned well-known ports that includes 273