Port 240: Reserved — A Well-Known Port Waiting for Purpose

Status: Reserved
Range: Well-known ports (0-1023)
Protocol: TCP/UDP
Assigned service: None

What Port 240 Is

Port 240 is Reserved. That's its official status in the IANA registry¹, marked since RFC 1060 in 1990². It sits in the well-known port range—the first 1024 ports that require root privileges to bind on Unix systems—but unlike ports 22 (SSH), 80 (HTTP), or 443 (HTTPS), port 240 doesn't carry anything.

No service runs on it. No protocol claims it. It's been held "just in case" for thirty-five years.

What Reserved Means

The Internet Assigned Numbers Authority (IANA) maintains three categories for ports³:

• Assigned — Currently allocated to a specific service or protocol
• Unassigned — Available for assignment if someone requests it
• Reserved — Held by IANA for special purposes, not available for regular assignment

Port 240 falls into that third category. Reserved ports are sometimes held at the edges of port ranges (like 0, 1023, 1024) to allow for future expansion of the port number system itself. Other times they're placeholders for protocols that were planned but never materialized.

Port 240 is just... reserved. The registry doesn't say why.

The Well-Known Port Range

Ports 0-1023 are called well-known ports or system ports. They're the ports where foundational Internet services live—the protocols that make the Internet work. About 76% of these ports were assigned when RFC 6335 documented the modern port assignment system⁴. That leaves roughly 24% either unassigned or reserved.

Port 240 is part of that 24%. A hole in the infrastructure. A number without a purpose.

What This Means in Practice

If you scan port 240 on a system, you probably won't find anything listening. Reserved ports aren't used in practice because:

1. No standard service runs there — Applications don't bind to port 240 by default
2. It's not available for assignment — You can't officially claim it for a new protocol
3. It's in the privileged range — Binding to ports below 1024 requires root/admin access on most systems

To check if anything is listening on port 240 on your system:

# Linux/macOS - using ss (modern)
sudo ss -tulpn | grep ':240'

# Linux/macOS - using netstat (traditional)
sudo netstat -tulpn | grep ':240'

# Using lsof
sudo lsof -i :240

You'll almost certainly see nothing⁵.

Why Reserved Ports Matter

The Internet's architecture includes breathing room. Not every port number needs to be assigned right now. Some are held in reserve for future protocols we haven't invented yet, for extensions to the port system itself, or simply as buffers in the numbering scheme.

Port 240 is part of that breathing room. It's been waiting since 1990—through the birth of the Web, the rise of HTTPS, the invention of WebSockets, the entire mobile Internet era—and it's still just... reserved.

Maybe someday someone will find a use for it. Maybe it will stay reserved forever. Either way, it's there in the registry: port 240, held in limbo, a number waiting for meaning.

Related Ports

• Port 22 — SSH, the secure shell that actually carries something
• Port 0 — Also reserved, represents "any available port" in some contexts
• Ports 225-241 — A block of ports in this range with various assignments and reservations