Variable Length Subnet Masking

Ok. So I've went over this with people, youtubed, etc. I get it, but I don't get it. And I really need to be able to do all of this without a calculator. I've studied tables, made charts and shit myself. Yeah it's not clicking. it's all there... just not clicking in place. 


  • Vague question so vague answer incoming.

    So say you're dealing with /24. You know you have a max of 254 hosts (8 bits minus broadcast and subnet address). If you have to subnet for 100 hosts, 35 hosts and 2 hosts, you start with the 100. Subtracting one host bit from the IP ( /25) gives you 128 addresses left, so you can subnet the 100 in that range ( - /25) which leaves you - open, so you can further subdivide that for the other subnets, like /26 for the 35 hosts (you're borrowing one more host bit so the maximum addresses here is 64 ( - /25). Etc etc until you run out of addresses or all your shit is subnetted.
  • I teach this. Will try to put something together for you when I'm home as there is an easy trick for calculating it in your head

    (Party): Mezghar says, "Stop."
  • It is seriously the only part of everything I've done with networking that has got me going What the fuck. And I already have a degree in the field. ffs.... I guess it's better than learning Calculus or physics though... 

  • @Sobriquet  Thanks. I'll need it. 

  • @Sobriquet  you home yet haha. Still looking stuffs. 

  • edited November 2015
    Oops! I forgot to say I was away all week so posting this from my phone! The 32bits are always the key (4x 8bit octets) together with remembering the 2^n calculations. 2^1 = 2 2^2 = 4 2^3 = 8 etc If you have a /25 mask you can do the following: 32-25 = 7 2^7 = 128 Which means the subnet is and Say you have a /26. 32-26 = 6 2^6 = 64 So you would have and Each time you go one down from the /24 you double the number of networks in that /24 range. So /25 is two network, /26 is 4 (see above), /27 is 8 etc. It works for the larger subnets too but you need to split the calculations for each octect and remember in the larger subnets (/23 etc) that x.x.x.0 can still be a usable address. Its not easy posting via the phone and I think it'll screw my formatting but take a look at: Sobby x Edit - Phone screwd the formatting as expected, sorry!

    (Party): Mezghar says, "Stop."
  • Oh and @Deladan so you know I've replied.

    (Party): Mezghar says, "Stop."
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