Port 1023 — The Last Well-Known Port — The Boundary That Protects

What Port 1023 Is

Port 1023 is not assigned to any service. It's the highest-numbered well-known port, which means it's the last of the ports reserved for system services. It marks a boundary—the edge where privilege ends and freedom begins. ¹

The Port Ranges

The Internet divides ports into three territories: ²

• Well-Known Ports (0-1023) — Reserved for critical system services. Binding to these requires administrative privileges (root on Unix-like systems). These are the ports for DNS, SSH, HTTP, HTTPS, SMTP, POP3, IMAP.
• Registered Ports (1024-49151) — Available for registered applications. Any user can listen here. This is where companies register custom services, but most applications simply pick a port and go.
• Dynamic/Ephemeral Ports (49152-65535) — For temporary, short-lived connections. Your client connections usually land here.

Port 1023 sits at the threshold between the first and second. That's its entire purpose.

Why Port 1023 Matters

Port 1023 exists because of a security decision made decades ago: Only root should be able to impersonate a system service. ³

On Unix-like systems, binding to ports 0-1023 requires administrative privileges. This prevents an unprivileged user from starting a fake SSH server on port 22 or a rogue DNS server on port 53. It's a simple, elegant security boundary. Port 1023 is where that protection ends.

If port 1023 didn't exist—if the boundary was fuzzy—an attacker could run arbitrary services as a regular user and potentially hijack critical network operations. Instead, the boundary is hard and clear: port 1023 is the last you can only touch as root.

Checking What's Listening

To see what's listening on port 1023 (if anything):

# On macOS or Linux
lsof -i :1023
ss -tlnp | grep 1023

# On Windows
netstat -ano | findstr :1023

Port 1023 is almost never used. Most systems have nothing listening there. It's the boundary, not the destination.

Unofficial Uses

While port 1023 has no official IANA assignment, some specialized systems (particularly z/OS NFS and certain network utilities) have used it for internal purposes. ⁴ These are exceptions. The port remains largely dormant, a sentinel marking the frontier.

Why Unassigned Ports Matter

Ports like 1023—officially unassigned—matter because they define the structure of the network address space. They're not vacant; they're reserved. They hold the shape of the system together. Some exist as boundaries. Some exist as future expansion space. Port 1023 is the former: a threshold, a line, a promise that root's world and user's world will never collide on the ports that matter most.