When using a Draytek Vigor router one or more users may receive the following message in their browser;

You have reached the maximum number of permitted Internet sessions.

Please close one or more applications to allow further Internet access.

Contact your system administrator for further information.

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This is due to the router reaching its maximum NAT sessions and is often caused by a rogue program on a device attached to your network keeping sessions open and not releasing them. The easiest fix is to simply restart the router, though that isn't a viable long term solution.

For example, the DV130 is able to handle up to 10,000 NAT sessions at a time before reaching the limit of its memory. Once it reaches that limit you will start to see the message described above. To be reaching that limit you might either have a large number of devices passing traffic through the router or a single device on your network creating too many sessions for some reason such as P2P sharing or some other poorly configured software (for example). Usual network traffic might create up to a couple hundred sessions at a time per device.

You can try this solution, after resetting the router and before more than half of its sessions are tied up, telnet to the unit using Putty or your preferred telnet tool and enter the following command:

portmaptime -t 600

This will reduce the lifetime of held open sessions to 10 minutes (in this example) and allow them to be recycled sooner.   The router might require a restart afterwards to ensure the setting is applied.

If making the portmaptime shorter and restarting the router makes no difference for you then you will need to either find the source of the excessive sessions or move to a router model with more memory to properly support a larger number of users such as the Draytek DV2860. 

If you are using a DV130 you could install an ethernet WAN router behind the DV130 and set the DV130 into PPPoE bridge mode.

For further info on the portmaptime command see the Draytek article here : How to adjust NAT session timeout value.