Port 263 is officially assigned to HDAP, a protocol so obscure that even the Internet doesn't remember what it does. In practice, the port has been quietly repurposed by Check Point and once carried Skype traffic—a reminder that official assignments and actual usage are two different things.
The Official Story
According to IANA's Service Name and Transport Protocol Port Number Registry, port 263 is assigned to "HDAP" for both TCP and UDP protocols.1 The contact listed is Troy Gau. That's it. No RFC. No protocol specification anyone can find. No explanation of what HDAP stands for or what problem it was meant to solve.
This is the Internet's fossil record. IANA maintains these registrations forever, creating a permanent ledger of every protocol that ever requested a port number. Some of these protocols went on to power billions of connections. Others vanished, leaving only a name in a registry.
Port 263 is one of the ghosts.
What "Well-Known" Actually Means
Port 263 falls in the well-known ports range (0-1023), also called system ports.2 These aren't called "well-known" because people know what they do—they're called that because IANA officially assigns them. It's a bureaucratic designation, not a popularity contest.
The well-known range was supposed to be reserved for foundational Internet services: HTTP on 80, SMTP on 25, DNS on 53. But the range also contains dozens of ports like 263, assigned to protocols that never achieved widespread use or have long since been abandoned.
What Actually Uses Port 263
The official assignment doesn't matter as much as you'd think. In practice, port 263 has been repurposed:
Check Point ClusterXL: Starting with R81.20, Check Point's firewall clustering software uses TCP port 263 for the CXLD daemon to perform Full Sync operations between cluster members.3 Earlier versions like R80.40 didn't use this port—it appeared in newer releases.
Skype (Historical): Port 263 UDP was one of the many ports used by Skype for voice and video communication.4 Skype was notorious for using whatever ports it could find to traverse firewalls, and 263 was part of that portfolio.
This is how the Internet actually works: official assignments are suggestions, but network engineers and software developers use what's available and what works.
The Mystery of HDAP
What was HDAP supposed to be? The searches turn up nothing substantial. No RFC defining it. No protocol documentation. No software that claims to implement it.
One possibility: there's an HDAP (HTTP Directory Access Protocol) used in some LDAP implementations to provide REST APIs over directory services.5 But whether that's the same HDAP assigned to port 263, or whether port 263's HDAP came first, or whether they're completely unrelated is unclear.
The Internet is full of these archaeological mysteries. Protocols get proposed, ports get assigned, and then the protocol never ships or gets replaced by something better. The port assignment remains, a bureaucratic ghost haunting the well-known range.
How to Check What's Listening on Port 263
If you want to see if anything is actually using port 263 on your system:
On Linux/Mac:
On Windows:
Most systems won't have anything listening on 263 unless you're running Check Point cluster software or certain VoIP applications.
Why Unassigned (and Forgotten) Ports Matter
Port 263 isn't technically unassigned—it has an official registration. But practically, it's abandoned. And that matters because:
The well-known range is finite. There are only 1,024 ports in the well-known range, and hundreds are assigned to protocols nobody uses anymore. This creates pressure to either reclaim unused assignments or let the bureaucratic fossil record accumulate.
Unofficial usage happens. When official assignments are unclear or abandoned, software developers repurpose ports for their own needs. This works until two different applications try to use the same port on the same system.
The registry is a historical document. IANA's port registry isn't just a technical specification—it's a record of what the Internet tried to become. Ports like 263 are reminders that for every HTTP and SSH, there were dozens of protocols that never made it.
Related Ports
- Port 262 — Arcisdms
- Port 264 — BGMP (Border Gateway Multicast Protocol)
- Port 389 — LDAP (if HDAP is related to directory services)
Frequently Asked Questions
The Real Lesson
Port 263 is a reminder that the Internet you use every day is built on layers of history. Some protocols succeed and become invisible infrastructure. Others fail and leave only a registry entry. And some, like HDAP, become ghosts—officially assigned but functionally forgotten, their port numbers repurposed by whatever software needs them next.
The bureaucracy says HDAP. The network says Check Point and Skype. The truth is that port numbers are just agreements, and when nobody remembers the original agreement, new ones get made.
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