Redirecting domain to sub domain

Firstly - are the IP's guaranteed to be static?

If so you build an IP -> subfolder lookup

$lookup = [
  '192.168.1.1' => '/folder1',
  '192.168.1.2' => '/folder2'
];

$ip = getIPAddress(); // Custom function see below

$destination = isset($lookup[$ip]) ? $lookup[$ip] : false;
if ($destination) {
  header('location: ' . $destination);
} else {
   // handle error
}

function getIPAddress(){
    if(!empty($_SERVER['HTTP_CLIENT_IP'])){
        //ip from share internet
        $ip = $_SERVER['HTTP_CLIENT_IP'];
    }elseif(!empty($_SERVER['HTTP_X_FORWARDED_FOR'])){
        //ip pass from proxy
        $ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
    }else{
        $ip = $_SERVER['REMOTE_ADDR'];
    }
    return $ip;
}

Select allOpen in new window

Note: The above code is untested but it demonstrates the concept.

The getIPAddress() function was obtained from here https://www.w3resource.com/php-exercises/php-basic-exercise-5.php other examples are here https://stackoverflow.com/questions/15699101/get-the-client-ip-address-using-php

Both of Julian's suggestions will work.

And if you have many users, your main budget may go toward dealing with visitor problems related to your IP routing, depends on your exact system.

Now the big question is what you hope to accomplish.

Keep in mind these days...

1) Many people use VPNs, so their IPs will constantly change.

2) Most people use cell devices, so their IPs may change as they bounce between cell towers + networks, depending on many factors.

5G is another consideration. No one knows yet, as implementations are rare + vary a great deal.

3) You can no longer make any Geo determination for a visitor on an IP, because of #1 + #2.

Suggestion: If required, open another question describing what you hope to accomplish + ask for design assistance.