Port 1660 sits in the registered ports range (1024-49151), officially assigned by IANA for a protocol you'll probably never encounter: skip-mc-gikreq—SKIP's multicast group key request protocol.1
What SKIP Was
SKIP (Simple Key-Management for Internet Protocols) was a cryptographic key management protocol developed in the mid-1990s, primarily by Sun Microsystems. It was designed to provide secure IP communications by managing encryption keys between network devices.2
Port 1660 specifically handled multicast group key management—the mechanism for distributing encryption keys to multiple recipients simultaneously in a multicast session. When a device needed to join an encrypted multicast group, it would use this port to request and receive the necessary cryptographic keys.
Why You Don't See It
SKIP lost. In the security protocol wars of the late 1990s, IPsec won. Modern networks use protocols like IKE (Internet Key Exchange) for key management, and SKIP faded into obsolescence.
The port remains registered in IANA's database, but finding something legitimately using port 1660 today would be remarkable—like discovering someone still using Gopher instead of HTTP.
The Registered Ports Range
Port 1660 belongs to the registered ports range (1024-49151). These ports are assigned by IANA to specific services upon application. Unlike well-known ports (0-1023) which require root privileges to bind on Unix systems, registered ports can be used by ordinary user processes.
Being registered doesn't mean the port is actively used—it means someone once requested it for a specific purpose. IANA doesn't revoke registrations when protocols become obsolete, so the registry is full of fossils like port 1660.
Security Considerations
Historical security databases note that port 1660 has been used by malware in the past.3 This makes sense—an officially registered but practically unused port is attractive to attackers precisely because legitimate traffic is unlikely and monitoring is rare.
If you see unexpected traffic on port 1660:
- It's probably not SKIP (unless you've time-traveled to 1997)
- It could be malware using an obscure port to avoid detection
- It might be a custom application that happened to pick this port
How to Check What's Listening
On Linux or macOS:
On Windows:
If something is listening and you don't know why, investigate. Legitimate SKIP usage is vanishingly unlikely.
Why Unassigned and Obsolete Ports Matter
The port number space is finite—65,535 ports per protocol (TCP and UDP). Every registered-but-unused port like 1660 represents a small piece of frozen history in the Internet's addressing scheme.
These ports also reveal how the Internet evolves: protocols are proposed, gain momentum, get assigned port numbers, then fade away when better solutions emerge. Port 1660 is a monument to SKIP—a protocol that tried to solve key management and lost to IPsec.
The registry never forgets. SKIP is gone, but port 1660 remains, waiting for requests that will never come.
Related Ports
- Port 500: IKE (Internet Key Exchange)—the modern key management protocol that replaced SKIP
- Ports 1661-1670: Various other registered ports from the same era, many similarly obsolete
Frequently Asked Questions About Port 1660
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