In what situations would it be necessary to consider network masks in addition to IP addresses when identifying visitors from a local network using PHP?

When identifying visitors from a local network using PHP, it may be necessary to consider network masks in addition to IP addresses if the local network uses subnetting. Subnetting allows a single IP address range to be divided into smaller subnetworks, each with its own network mask. To accurately identify visitors within a specific subnet, you need to compare the visitor's IP address with the subnet's network mask to determine if they belong to that subnet.

// Get the visitor's IP address
$visitor_ip = $_SERVER['REMOTE_ADDR'];

// Define the subnet's IP address and network mask
$subnet_ip = '192.168.1.0';
$subnet_mask = '255.255.255.0';

// Calculate the network address using bitwise AND operation
$network_address = long2ip(ip2long($visitor_ip) & ip2long($subnet_mask));

// Compare the network address with the subnet's IP address
if ($network_address == $subnet_ip) {
    echo 'Visitor is from the local network subnet.';
} else {
    echo 'Visitor is not from the local network subnet.';
}