Port 1804: Unassigned — Empty Space in a Crowded Map

What Range This Port Belongs To

Port 1804 falls in the registered port range: 1024 to 49151.

This range is the middle ground of the port map. Below it, ports 0 through 1023 are the well-known ports — HTTP at 80, HTTPS at 443, SSH at 22. Those require root or administrator privileges to open on most operating systems. Above 49151 are the ephemeral ports, used temporarily by your OS for outbound connections and never permanently assigned to anything.

The registered range is where applications stake claims. A developer builds a protocol, submits paperwork to IANA, and if approved, a port number becomes theirs. MySQL lives at 3306. PostgreSQL at 5432. Redis at 6379. Each of those was once just a number waiting to be claimed.¹

Port 1804 is still waiting.

No Official Assignment

IANA's registry lists port 1804 as unassigned for both TCP and UDP.² No protocol has claimed it. No RFC defines behavior for it. It's empty space in a very crowded neighborhood.

Some port databases note it has appeared in connection with suspicious activity in the past — the same way any unused port might show up in a scan or an old malware report. That's not a characteristic of port 1804 specifically. It's what happens to any door that's occasionally found open with nothing legitimate behind it.

How to Check What's Listening on This Port

If you see traffic on port 1804 and want to know why:

On Linux or macOS:

# Show what process has port 1804 open
sudo lsof -i :1804

# Or with ss (Linux)
sudo ss -tlnp sport = :1804

On Windows:

# Show connections on port 1804
netstat -ano | findstr :1804

# Then look up the PID
tasklist | findstr <PID>

If something is listening on port 1804 and you didn't put it there, that's worth investigating. Unassigned ports don't have legitimate well-known occupants, so any process using one is either a custom application you know about, something you should know about, or something you definitely want to know about.

Why Unassigned Ports Exist

The registered range contains 48,127 port numbers. Not all of them are filled. Some were assigned and then abandoned as protocols fell out of use. Some were never assigned at all.

This gap-filled map matters for a few reasons. Firewall rules often work by allowlisting known ports and blocking everything else — unassigned ports are easier to block without worrying about breaking something legitimate. Security scanners use the gaps to detect unexpected services. And occasionally, a developer building something new picks an unassigned port number before filing for official registration, which is why traffic on unassigned ports isn't automatically malicious.

Port 1804 is simply unclaimed. The port system has room for it, IANA maintains the space, and it waits.