geo_ip_set_header.conf

http {

    geo $subnet {
        default         0;
        45.55.0.0/24    1;
    }

    map $subnet $value {
        default         "";
        1               $remote_addr;
    }
}

server {
    ...
    proxy_set_header Subnet $value;
    proxy_pass  http://upstream;
}

This configuration can be used to determine a clients address within a specific subnet and add a header with the $remote_addr variable within NGINX