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

LogomachistLogomachist Registered Users Posts: 3 ■■■□□□□□□□
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?

The correct answers listed are 0, 254 and 255.

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


  • omi2123omi2123 Member Posts: 189
    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. then ur subnet mask will be 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
  • aftereffectoraftereffector Member Posts: 525 ■■■■□□□□□□
    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.
    CCIE Security - this one might take a while...
  • [Deleted User][Deleted User] Senior Member Posts: 0 ■■□□□□□□□□
    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.
  • LogomachistLogomachist Registered Users Posts: 3 ■■■□□□□□□□
    Ah OK, I get it now. Thanks!
  • stryder144stryder144 Member Posts: 1,684 ■■■■■■■■□□
    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.
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