Port 548: AFP
Port 548 is AFP, Apple's old file-sharing protocol, used by older Macs, Time Capsules and NAS boxes.
What runs on port 548
The Apple Filing Protocol served Mac file sharing over TCP 548 for decades and was long the standard for Time Machine network backups. Apple has since moved to SMB, macOS no longer offers AFP when sharing files, and Apple has deprecated it.
On a home network
AirPort Time Capsules and older NAS boxes with AFP enabled for Time Machine listen on 548. Recent Macs do not.
If port 548 is reachable from the internet
AFP should never be reachable from the internet. It exposes file shares to password guessing, and the devices that still run it, such as Time Capsules, no longer receive security updates.
How to check whether it is open on your network
On a NAS, check the file services settings and switch Time Machine to SMB if possible. Confirm no router rule forwards 548.
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