子网掩码前缀长度换算,如何计算掩码的网络前缀

<?php
$len=0;
$netmask="255.255.255.0";
$split_mask=explode('.',$netmask);
foreach($split_mask as $v)
{
    $len+=8-log(256-$v,2);
}
echo $len;
?>