I am using a TP-link router and I want to restrict use of WiFi for some specific user via MAC address. How do I do this?
The easiest way would be acessing the router's config page with 192.168.1.1 (default user and password are usually admin and 1234) and allowing only the devices you use from the MAC filtering tab/section (like filling a list of devices that are allowed to use the router). This will deny the access to any device with a mac not specified on the mac filtering list.
access your routers webinterface, usually at http://192.168.1.1
login with the user admin and the password you have set during the initial setup
go to Wireless > Wireless MAC Filtering
* on older devices MAC filtering may be listed under Firewall
BEFORE enabling the filter make sure to carefully read through the settings and add your computers (WiFi) MAC address. Otherwise you may end up locking yourself out and you'd have to use a cable to access and change the configuration.
On newer devices the rule you want to set is "Deny the stations not specified by any enabled entries.."
On older devices the setting is reversed, "Allow these PCs with enabled rules to access the internet."
Software captive portals
The captive portal technique forces an HTTP client on a network to see a special web page (usually for authentication purposes) before using the Internet normally. A captive portal turns a Web browser into an authentication device
Enable MAC Address Filtering on Wireless Access Points and Routers
Connection slow? - How to block MAC address - TP-Link or other routers
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How to configure MAC address Filtering on TP-LINK 11G Router