Variable Length Subnet Masking

Deladan

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. 

Kafziel

Vague question so vague answer incoming.

So say you're dealing with 10.0.0.0 /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 (10.0.0.0 /25) gives you 128 addresses left, so you can subnet the 100 in that range (10.0.0.0 - 10.0.0.127 /25) which leaves you 10.0.0.128 - 10.0.0.255 open, so you can further subdivide that for the other subnets, like 10.0.0.128 /26 for the 35 hosts (you're borrowing one more host bit so the maximum addresses here is 64 (10.0.0.128 - 10.0.0.192 /25). Etc etc until you run out of addresses or all your shit is subnetted.

Sobriquet

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

Deladan

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... 

Deladan

@Sobriquet  Thanks. I'll need it. 

Deladan

@Sobriquet  you home yet haha. Still looking stuffs. 

Sobriquet

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 192.168.0.0/25 and 192.168.0.128/25. Say you have a /26. 32-26 = 6 2^6 = 64 So you would have 192.168.0.0/26 192.168.0.64/26 192.168.0.128/26 and 192.168.0.192/26. 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:http://www.dslreports.com/faq/cisco/30.1_Quick_subnet_calculating_techniques Sobby x Edit - Phone screwd the formatting as expected, sorry!

Sobriquet

Oh and @Deladan so you know I've replied.