Subnetting & VLSM

Basset Hound · New Member Joined: 23 Sep 2011 Posts: 2 Location: home

Hey Everybody,
I have something I would like to share. I am just starting to study for my CCNA, and like others found subnetting difficult. I searched the Internet for an easier way, some I liked, some I didn't. I have a method I use now that not only made it easier for me but also helped me understand VLSM. I have not seen this method anywhere, but I am not an expert (yet). I would like to ask your collective opinion about it, if I may.

Mask------ Bit----------Group---------Prefix
128--------1----------- 128------------/9 /17 /25
192--------2----------- 64-------------/10 /18 /26
224--------3----------- 32-------------/11 /19 /27
240--------4-----------16-------------/12 /20 /28
248--------5----------- 8--------------/13 /21 /29
252--------6----------- 4--------------/14 /22 /30
254--------7----------- 2--------------/15 /23 /31*
255--------8-----------------------/8 /16 /24

31* - point to point connections only
32 - invalid

It was helpful for me to think about subnets as groups, like in the chart above, for example /25 is two groups of 128 (0-127 & 128-255) so weather I have the mask or prefix I can quickly determine the group range using simple math. Another example; 192.168.134.109/21 I know the mask is 248 working on the third octet and the group range is 8, so in this example the first divisible multiple of 8 is 128 so the network number is 192.168.128.0 and the broadcast is 192.168.135.255 and total hosts = 2046 (8x256-2). Once I started thinking about subnets as groups VLSM fell into place. Using the two groups of 128 as an example /26 could now subdivide one of the two groups into a smaller group of 64 (0-63). So as an example 192.168.100.0/25 could be 192.168.100.0 to 192.168.100.127 & 192.168.100.128 to 192.168.100.255, using one group to further subnet you could have 192.168.100.0/26 that would be 192.168.100.0 to 192.168.100.63, you could keep subnetting as long as the groups add up. Has anyone seen this before?

Headhunter · **Post subject:** **Posted:** Thu Nov 10, 2011 3:03 am

for subnetting there is a faster way
example /28 or .240
256 - 240 = 16
subnet1 x.x.x.0
subnet2 x.x.x.16
subnet3 x.x.x.32
etc etc

broadcast is one less then next subnet
subnet2 broadcast = subnet3 x.x.x.32 - 1 = x.x.x.31
_________________
Koen
CCSI #30501 CCNP CCIP CCVP CCSP CCDP Cisco Advanced Wireless LAN Field Specialist Cisco Unity Design Specialist Cisco IP Telephony Design Specialist JNCIS-ES JNCIA-ER JNCIA-EX MCSE MCSA:Messaging

Basset Hound · New Member Joined: 23 Sep 2011 Posts: 2 Location: home

Yeah, i've seen that method, it's in all the books, i don't think it's faster and it doesn't explain VLSM. With the group method you can quickly calculate multiples of ranges. Also for VLSM there can be 2 groups of 128 or 1 group of 128 and 2 groups of 64 or 2 groups of 64 and 4 groups of 32, as long as it adds up to 256.