Port 445: SMB
Port 445 is SMB, the protocol Windows, Macs and NAS boxes use for shared folders and printers.
What runs on port 445
Since Windows 2000, SMB runs directly over TCP 445 without the older NetBIOS layer on 139. It carries file shares, printer shares and a lot of Windows domain traffic. Samba provides the same service on Linux and most NAS devices, and macOS uses SMB for its own file sharing.
On a home network
Windows PCs, NAS boxes, Macs with File Sharing on, routers sharing a USB drive, and some media players listen on 445. Seeing it on those devices inside your network is normal.
If port 445 is reachable from the internet
Port 445 should never be reachable from the internet. In 2017 the WannaCry ransomware worm used the EternalBlue exploit against SMB on 445 to spread around the world in a day, and NotPetya used the same exploit weeks later. Many ISPs block 445 for this reason, but a forwarded port or a device sitting in the router's DMZ can still expose it, and exposed shares are also brute forced and scraped for data.
How to check whether it is open on your network
Make sure no port forwarding or DMZ rule on your router points at a PC or NAS. RECON's Exposure Check tests whether 445 answers on your public address, and a LAN port scan shows which devices share files internally.
To find out whether port 445 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.