Transform a wireless router into a switch

 To use an old wireless router as a switch to connect more devices, loosely follow these steps:

1) Connect router to network as a router (via the WAN port), temporarily.

2) Find its IP and via a browser connect to the router admin interface (depends on vendor).

3) Disable Wireless functionality, instructions depend on vendor. Example for DLink

4) Disable DHCP for router, instructions depend on vendor, look somewhere in LAN settings.

5) Save settings and reboot router.

6) Connect the "internet" cable to one of the LAN ports instead of WAN port, consider WAN port as "dead" now.

7) If it worked, all devices connected to the new switch will have IPs assigned by the main router.

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8) If you need to change settings, reconnect via the WAN port again to access admin interface.