Port 1604 sits in the IANA registry with an official assignment, but its day has passed. This port was registered for Citrix ICA (Independent Computing Architecture) browsing—a mechanism that helped clients discover available applications and servers on the network. Modern Citrix deployments have moved on, leaving port 1604 as a registered ghost.
What port 1604 was for
In older Citrix environments (MetaFrame and WinFrame), clients needed to find which servers were available and what applications they could run. Port 1604 provided the answer through a broadcast mechanism.
When an ICA client wanted to browse available applications, it sent UDP broadcast packets on port 1604. These packets shouted across the local network, asking "who has applications for me?" The master browser or data collector for the Citrix farm would respond, providing a list of available resources.
For WinFrame and MetaFrame 1.8 (before Feature Release 1), port 1604 was the only option. You had no choice—ICA browsing meant UDP port 1604.1
Why it was deprecated
Broadcasting works, but it doesn't scale well and creates security concerns. Every client shouts into the void hoping someone answers. As networks grew larger and security requirements tightened, Citrix moved away from this approach.
Starting with MetaFrame XP, Citrix discouraged the use of UDP port 1604 and broadcast-based browsing entirely. The new architecture used TCP-based mechanisms on ports 80 and 443—the same ports that carry web traffic. This meant ICA traffic could traverse firewalls more easily and benefited from HTTPS encryption.2
Port 1604 still exists in the IANA registry, officially assigned to Citrix. But if you're running modern Citrix systems, you're almost certainly not using it.
What range port 1604 belongs to
Port 1604 falls in the registered ports range (1024-49151). These ports are assigned by IANA to specific services upon request, but unlike well-known ports (0-1023), they don't require administrator privileges to use on most systems.
The registered range is where you find ports for specific commercial applications, enterprise software, and specialized protocols. Port 1604 was registered by Brandon Pederson for Citrix's ICA browsing functionality.3
Being in the registered range means:
- The port has an official assignment and documented purpose
- Applications should check IANA before using this port for other purposes
- On most systems, any user process can listen on this port
- Firewalls might have it blocked by default since it's not commonly used anymore
Checking what's listening
If you want to see whether anything is actually using port 1604 on your system:
On Linux or macOS:
On Windows:
If you find something listening on port 1604, it's likely one of two things:
- A legacy Citrix installation that hasn't been updated
- Some other application repurposing the port (less common since it's officially assigned)
Why unassigned ports matter
Port 1604 is technically assigned, but functionally abandoned. This raises an interesting question: what happens to registered ports when technology moves on?
Unlike IP addresses, which get reclaimed and reused, port numbers tend to stay in the registry even when nobody uses them anymore. Port 1604 will probably remain "Citrix ICA browsing" in the IANA database indefinitely, even though Citrix itself has moved on.
This matters because:
- It documents the history of network protocols
- It prevents conflicts—new applications won't accidentally choose 1604 and collide with legacy Citrix systems
- It shows how protocols evolve—the move from UDP broadcasts to TCP connections reflects broader trends toward security and scalability
Every deprecated port tells a story about what didn't scale, what wasn't secure enough, or what simply got replaced by something better. Port 1604's story is about the shift from broadcast-based discovery to directed, encrypted connections.
Related ports
- Port 1494 — Citrix ICA protocol (the actual application traffic, still in use)
- Port 2598 — Citrix ICA browsing (another legacy browsing port)
- Port 80/443 — HTTP/HTTPS (where modern Citrix traffic actually flows)
Frequently asked questions about port 1604
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