Why can't you use 1 or 253 as a subnet mask?

Logomachist

I'm taking a practice test that came with Mike Myers' All-in-One Comptia Network+ text, and while I think I understand the basic idea behind network masks I'm stumped by the following question:

Which of the following are valid numbers you can use for a subnet mask?
1
254
255
256
253

The correct answers listed are 0, 254 and 255.

Why can't you use 1 or 253 as a subnet mask? :

omi2123

well, it goes like this;....

128 192 224 240 248 252 254 255

128 64 32 16 8 4 2 1

the bottom line is ur octets & the top line r ur all possible subnet masks.......if u have a network id i.e. 192.168.10.32/27 then ur subnet mask will be 255.255.255.240....look at the last octet on ur subnet mask..../27 means ur forth octet will be anything other than 255.......if u draw the above mentioned chart, then u can see how easily /27 (8+8+8+3) falls into the third column under 224 mask......hope it helps

aftereffector

Your subnet mask will be all binary ones on one side and all zeros on the other. 1 in binary is 00000001, so that's out - you can't have a 1 after your zeroes. 254 is 11111110, so that works. 255 is 11111111, which also works. 256 is too many numbers. 253 is 11111101, which is also not a valid subnet mask.

[Deleted User]

256 is not a valid subnet mask because each IPv4 address octet consists of 8 bits. (128,64,32,16,8,4,2,1) If you have all of those bits turned on, you would have 255 (11111111). 256 would have you rollover into a new subnet.

Logomachist

Ah OK, I get it now. Thanks!

stryder144

What Aftereffecter and kMastaFlash have said is spot on. Also, remember that when counting, we start with 0. Thus, 0-255 is the equivalent of 1-256.