Port 3308: Unassigned — MySQL's Favorite Backup Address

Port 3308 has no official service assigned by IANA. It sits in the registered port range, where ports 1024 through 49151 live — numbers that require no special privileges to use, and that IANA maintains a registry for, but hasn't assigned here.

Which means anyone can use it for anything. And in practice, they do.

The Registered Port Range

The Internet's port space is divided into three zones:

• Well-known ports (0-1023): Reserved for major protocols. HTTP lives at 80. SSH at 22. These require root or administrator privileges to bind.
• Registered ports (1024-49151): IANA maintains a registry here, but registration is optional. Vendors register ports for their software, but most of the ~48,000 numbers are simply unclaimed.
• Ephemeral ports (49152-65535): Used temporarily by your operating system for outbound connections. Your browser uses one of these when it connects to a web server.

Port 3308 is a registered port with no registered service. It's open real estate.

What Actually Runs Here

Port 3308 has become a de facto alternate MySQL port, one step removed from the defaults.

The story goes like this: MySQL's official port is 3306.¹ When someone needs to run a second MySQL instance on the same machine — a separate database for testing, a legacy version alongside a new one, a Docker container mapped to the host — they need a different port. Port 3307 is the first choice, often claimed by MariaDB (MySQL's open-source fork). Port 3308 is the next stop.

This pattern shows up everywhere:

• WAMP Server on Windows, when running both MySQL and MariaDB simultaneously, assigns 3307 to the non-default engine and sometimes 3308 to a third instance.²
• Docker deployments commonly map port 3308 on the host to port 3306 inside a container, letting two MySQL containers coexist on one machine.³
• MySQL's own documentation on running multiple instances explicitly uses 3307 and 3308 as example ports.⁴

None of this is official. It's convention built from repetition — enough developers reached for the same answer that it became the expected answer.

How to Check What's Listening Here

If you see traffic on port 3308 and want to know what's using it:

On macOS or Linux:

lsof -i :3308

On Linux (alternate):

ss -tlnp | grep 3308

On Windows:

netstat -ano | findstr :3308

The output will show you the process name and ID. If it's mysqld, that confirms the common case. If it's something else entirely, that's not unusual either — unassigned ports are used for all kinds of internal services, development servers, and proprietary software.

Why This Port Exists Unused

The registered port range has roughly 48,000 slots. Fewer than 10,000 have registered services. The rest aren't wasted — they're available. The design decision to leave ports unassigned gives the ecosystem room to breathe, letting conventions emerge organically rather than being centrally planned.

Port 3308 is a small example of that working correctly. No committee decided it would be the MySQL backup port. Enough people with the same problem made the same choice, and consensus formed.