I am trying to build a class C checker in php (it's part of a larger project). But just wondering if someone could clarify what needs to be checked? Is it just the third octet of the IP that must be different? --as I understand this doesn't always mean it's on a separate network. Also as I understand it there are 5 networks classes (a,b,c,d,e), are the other networks ignored... Or am I getting the wrong end of the stick here?