is 255.255.255.40 valid?

[greenhand]

Hi,

I had an telephone job interview and one of the question asked was how many hosts can have with a network mask of 255.255.255.40. is it a valid mask? as far as I know, there should not be zero in the middle of the 1's ..? thanks a lot for help

 

[deeno]

You are correct, that is an invalid subnet mask... You cannot have a zero in between a 1 and another 1. The one digits go from left to right. Possible subnet masks would be like:

10000000 = 128
11000000 = 192
11100000 = 224
11110000 = 240
11111000 = 248
11111100 = 252
11111110 = 254
11111111 = 255

So, a subnet mask of 255.255.255.0 would be:

11111111.11111111.11111111.00000000

And a subnet mask of 255.255.240.0 would be:

11111111.11111111.11110000.00000000

Hope that helps. You should be able to find plenty of info on this topic on the net...

deeno