Oops! Make that: 2^n - 1 > 10 2^n > 11 It helps to actually add one to both sides. ;-) James Lee Bell wrote: > > 2^n -1 > 10 ---> 2^n > 10 ---> (1,4,8,*16*,32,64,...) > 10, so n > = 4 and the mask is 255.255.255.240 as Entelin stated. >