server_name *.hostname.com;

if ($host ~* ^([0-9]+).hostname.com$) {
    set $proxyhost 192.168.56.$1;
}

proxy_pass http://$proxyhost;

server_name ~^(?<subnum>[0-9]+).hostname.com$;

proxy_pass http://192.168.56.$subnum;

map $host $backend {

    default 1;
    ~*^(?P<number>[0-9]+).hostname.com$        $number;

    # FIXME: [0-9]+ must be replaced to regex with accurate check 1..254 range
    # for example [1-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-4] or similar

}

server {
    server_name *.hostname.com;

    location / {
        proxy_pass http://192.168.56.$backend:80;
    }

}