What is port 137-139?

Port 137-139 is a well-known port used by NetBIOS over TCP/UDP. Legacy Windows file/printer sharing and name service. A port is like an apartment number at an IP address: it tells incoming traffic which service on the device it belongs to. See the full common ports reference for how this fits with other services.

How to forward port 137-139

  1. Give the device running NetBIOS a fixed local IP (a static IP or a DHCP reservation) so its address never changes.
  2. Open your router's admin page and find Port Forwarding.
  3. Create a rule forwarding TCP/UDP port 137-139 to that device's local IP.
  4. Save, then test from outside your network.

Walk through it in full with how to set up port forwarding, and plan a stable address with the DHCP pool planner.

Is opening port 137-139 safe?

Forwarding any port exposes a service to the internet, so only open port 137-139 if you need it, point it at a single device, and keep that device updated. Read is port forwarding safe? before you open anything.