The simplest explanation I can think of. Imagine a blank canvas of your house. Your router gives off a big circle that’s your WiFi.
An extender add another big circle wherever you put it, but only communicates to your router and a device hooked up.
You add a mesh network, you just added x amount of circles to your network, that are all interconnected and connect/ talk with each other.
Even more TLDR: It’s like a circle vs a Venn diagram, vs a 5 circle Venn diagram, kind of.
Latest Answers