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.
>