Symptoms
- A game, security camera, or server can't be reached from outside the home network
- Port checker tools report the port as closed even after setting up forwarding
- Forwarding worked before and stopped after a router change or restart
- Works on local network but not from outside
Likely causes
- CommonDevice's internal IP address changed since the rule was set upPort forwarding rules point to a specific internal IP, which can change if the device doesn't have a fixed address.
- CommonDouble NAT from an ISP modem-router plus a personal routerIf the ISP's device is also routing traffic, forwarding needs to be set up on it too, or it should be put in bridge mode.
- PossibleISP blocking the specific portSome providers block certain ports (like 80 or 25) for residential accounts.
- PossibleDevice's own firewall blocking the port internallyEven with forwarding set up on the router, the device itself may block incoming connections.
- PossibleTypo in the port number or protocol (TCP vs UDP)Forwarding rules must match the exact port and protocol the application uses.
What to do
- Assign a static (fixed) internal IP address to the device you're forwarding to, either through the router's DHCP reservation feature or the device's own network settings.
- In the router's port forwarding section, confirm the internal IP, port number, and protocol (TCP, UDP, or both) match what the application requires.
- Check whether you have two routers in sequence (ISP modem-router plus your own router); if so, either forward the port on both or set the ISP device to bridge mode.
- Confirm your public IP address using a 'what is my IP' site, then use an online open port checker tool to test the specific port from outside.
- Temporarily disable the device's own software firewall to test if that's blocking the connection, then re-enable it and add a specific exception if that fixes it.
- Contact your ISP to ask if the port you need is blocked at their network level, which is common for ports like 25 or 80 on residential plans.
- Restart the router after saving forwarding rules, since some routers need a reboot to apply them.
Tools and parts
- Computer for router admin access
- Online open port checker tool
When to call a pro
If double NAT or ISP-side blocking is confirmed and you're not comfortable adjusting modem settings, an IT professional or your ISP's technical support can assist.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.