I suspect because public IP isn't the same thing as WiFi. You could have multiple public IPs on any WiFi network. Or even multiple SSIDs per public IP. Where as the SSID is more analogous with "WiFi" (at least in terms of what the average user might understand).
The problem with SSIDs is they're not globally unique. So you need some way of identifying one SSID from another with the same name. Hence why they store both.
To be honest, once you go the app route, the best approach would likely be ssid and MAC (or the phone ID). IP is way too volatile to be useful - what happens when the wifi network reassigns it to someone else?
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I suspect because public IP isn't the same thing as WiFi. You could have multiple public IPs on any WiFi network. Or even multiple SSIDs per public IP. Where as the SSID is more analogous with "WiFi" (at least in terms of what the average user might understand).
The problem with SSIDs is they're not globally unique. So you need some way of identifying one SSID from another with the same name. Hence why they store both.
To be honest, once you go the app route, the best approach would likely be ssid and MAC (or the phone ID). IP is way too volatile to be useful - what happens when the wifi network reassigns it to someone else?