String together the 4 octets of the IP addy as if they were one binary number. Or for the procedurally-minded: take the first number of the dotted-quad times 2

String together the 4 octets of the IP addy as if they were one binary number. Or for the procedurally-minded: take the first number of the dotted-quad times 256^3 plus second times 256^2 etc etc etc.