Subnet basics: Do you need separate Wi-Fi subnets in a single building?
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Setting up a wireless LAN (WLAN) is more of an art than a science. The answer to your question depends on how many people you need to support in a given area. In a single building, for example, if you have a lot of people accessing the WLAN full time, you probably want to make each floor a subnet so you can segregate your traffic for better troubleshooting. On the other hand, you have to make sure there's a handoff to go from floor to floor.
You could also set up an entire building as one virtual access point (AP), making the entire building a subnet. In practice, however, doing that can give you bad performance. Wi-Fi isn't the same as cellular, where sites are more intelligent and have direct control of sessions to manage them across cell sites. Wi-Fi is more like an FM radio where the mobile device locks onto the AP that has the strongest signal. If you have two access points, and they are at exactly the wrong distance between each other, you could have a signal but get interference, creating dead zones.
There are also a lot of variables when sizing APs for geographical coverage. It depends how much metal is in your building and how many cubicles or rooms you need to reach into. And you have to know how many devices will access the WLAN. It could easily be three per person, with smartphones, tablets and laptops. Based on the standards, you can logically control up to 255 devices per AP, but, in practice, APs don't handle that many.
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