5 Replies Latest reply: Apr 16, 2012 12:37 AM by RG_WB

# ip subnetting part 1 cbt nuggets

7  6   5   4  3 2 1 0 bits

128 64 32 16 8 4 2 1 powers

128 192 224 240 248 252 254 255

if i wanted 4,5,6 or 7 subnets i find out how many bits it takes .

100=4

101=5

110=6

111=7

3 bits is 128+64+32= 224

now if i wanted 8 subnets 1000 which is 4 bits

128+64+32+16=240 which is wrong as 2^3=8 . 224

on cbt nuggets subnetting video he says to use the number of bits used to make up the number of subnets you need.

That's fine as long as you don't have a number which lands on a power of 2.

So do i use the powers all the time instead of the way he said by using the number of bits?

• ###### 1. Re: ip subnetting part 1 cbt nuggets

8 subnets with 1000 hosts per each subnet?

you start with number that gets you 1002 hosts when raised to power of 2.

that 2 is there 'cause we subtract 2 from number of hosts.

this way we get the number of bits used for host portion;

similarly ,then u have to figure out number for subnets;

which is 3 bits.

if number does not fall on "power of 2", get next one over so u have some extra hosts or subnets.

================

I see;

8 subnets is 3 bits are used; couting from left, 3rd position is 224. rest of bits (5 for C class) are left for host part.

To get number of hosts, you start countign from right.

I think he uses The table is used to count position to spot mask.

It has been a while since i watch his videos.

Message was edited by: Martin

• ###### 2. Re: ip subnetting part 1 cbt nuggets

I know about the hosts part which is always to the power of 2 minus 2

But in the video he starts off the section about subnetting by saying you use the number of bits to find the subnet mask. If i wanted 5 subnets he says it takes 3 bits to make up the number 5. So you go along 3 128-64-32 and it lands on 32

so your subnets will go up by 32 and the mask is 224

Later on in the video he says you need 100 networks.

100 takes 6 bits to make up.

but if you wanted to find out exactly how many networks that was you use the formula 2^n power. Should i just use that all the time as to not get confused in my exam?

• ###### 3. Re: ip subnetting part 1 cbt nuggets

6 bits is not enough; you need 7 for 100 subnets;

I think you may have to watch video again.

• ###### 4. Re: ip subnetting part 1 cbt nuggets

Well spotted. But the question still stands do i just stick with powers of and nevermind what he said in the first part of the video?

• ###### 5. Re: ip subnetting part 1 cbt nuggets

Find the method that works best for you. I used CBTN, the CCNA OCG video, and other resources and subnetting quizes along the way. I understood the concept but didn't have that "aha" moment until I watched the video; coupled with my bit counting on my fingers and much practice, got me to the point where I can subnet in my head now and get the right answer quickly.  So if X^2 works for you everytime and quickly stick with that bro' and tweek and tune as needed when you practice.