Supernetting

Hi All,

When you are summerizing routes - do you write it all out in binary as per all the articles I've read state or does there come a point where you can do it in your head?

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matt333

Pretty much.. you start to realize that the left most bit has a range for XX to XX.. if the the address is in that range then it is summarized by that address... as you study it will become easier

Roguetadhg

control wrote: »

Hi All,

When you are summerizing routes - do you write it all out in binary as per all the articles I've read state or does there come a point where you can do it in your head?

Write it out, it saves the cost of making a mistake

You can get that good to do it in your head. To get that good takes a hell of a lot of writing it out.

boredgamelad

I developed my own, or at least I think it's my own--if I was ever taught to supernet this way, I don't remember--method for supernetting that doesn't require conversion to binary. I imagine it's only useful to me because of the way my brain works, but I find supernetting to be about as simple as subnetting now. Can't do everything in my head quite yet though

Roguetadhg

Whats your method? It might be good for other people as well

fsanyee

Practice until you can do it in your head.

control

boredgamelad wrote: »

I developed my own, or at least I think it's my own--if I was ever taught to supernet this way, I don't remember--method for supernetting that doesn't require conversion to binary. I imagine it's only useful to me because of the way my brain works, but I find supernetting to be about as simple as subnetting now. Can't do everything in my head quite yet though

If you could provide some examples of your method that would be helpful, maybe help some of us who are just starting out.

If anyone else also cares to show us their own methods....the stage is yours..

boredgamelad

I am eyeball deep in ICND2 study right now but I'll be passing it on Friday... so I'll write up something this weekend, if I can find the time.

xbuzz

This thread is the best explanation ever of subnetting tbh. If you can get this down in your head then you can do an sub/supernetting in your head, in under 20 seconds. I used this method and I didn't have to write anything down on sheets in CCENT, it's so easy.

http://www.techexams.net/forums/ccna-ccent/38772-subnetting-made-easy.html

I do something slightly different though. I have this tabled memorised in my head...

128

1
8

-4

1

If i get subnet of say x.x.x.x / 19

What I do. I knw 24 is nearest full octet. So i take 19 from 24 = 5

(What I mean by the nearest full octect, in IP address you have 4 octects 1.2.3.4 1st = 0-8, 2nd = 8-16, 3rd = 16-24, 4th = 24-32, so if you have a prefix /19, you know that is in the 16-24, or the 3rd octect, so you subtract it from 24 then add 1, as above.)

Then I add 1 to the answer, that gives me 6.

I go to my memorised table and count 6 from left which is 32, so that's my network size.

If i want my mask, I just subtract 32 from 256 = 224 (255.255.255.224)

Other examples

/31 (32-31 = 1) (1+1=2) (2 on table = network of 2) (Mask is 256-2 = 254 .....255.255.255.254)
/30 (32-30 = 2) (2+1=3) (3 on table = network of 4) (Mask is 256-4 = 248 .....255.255.255.252)
........

/11 (16-11 = 5) (5+1=6) (6 on table = network of 32) (Mask is 256-32 = 224 ....255.224.0.0)

........

/4 (8-4 = 4) (4+1 = 5) (5 on table = network of 16) (Mask is 256-16 =240 ...240.0.0.0)

........

Summarizing just uses the same concept. Pick the smallest size network from the table

128 64 32 16 8 4 2 1

...that encompasses the networks you need to summarize.

If we pick 64, we know that that is 7 from our memorised table, but we're going the opposite way so we need to take 1 away ( 7-1= 6) So we'd take that 6 from whatever octect we're working from, which will be either 32, 24, 16, 8. (32-6 = /26, 24 - 6 = /18 etc)

What I wrote there is probably nonsensical, as I don't really think about it when I do it in my head, but that thread I linked is worth it's weight in gold.

Ltat42a

Found this quick method, seems to work pretty good. Here's an example -

10101100.00010000 = 172.16
10101100.00010001 = 172.17
10101100.00010010 = 172.18
10101100.00010011 = 172.19
10101100.00010100 = 172.20
10101100.00010101 = 172.21
10101100.00010110 = 172.22
10101100.00010111 = 172.23
10101100.00011000 = 172.24
10101100.00011001 = 172.25
10101100.00011010 = 172.26
10101100.00011011 = 172.27
10101100.00011100 = 172.28
10101100.00011101 = 172.29
10101100.00011110 = 172.30
10101100.00011111 = 172.31

1. How many subnets are in the range? The RFC1918 Class B range is 16 subnets.

2. What power of 2 equals our range? 16 subnets = 2(4) so the answer is four.

3. Subtract the figure from step 2 from the default mask of our address range. In this example our default mask is 16 so the mask after subtracting 4 is /12.

4. Add this mask to the first address in the range - 172.16.0.0/12 in this example

boredgamelad

So the method I developed isn't unique--honestly, I didn't think it was. What Ltat42a just posted is essentially what I do, but my exact method is a bit different. I still don't remember ever specifically learning to supernet, though. I think it just came naturally to me after learning subnetting.

mella060

Roguetadhg wrote: »

Write it out, it saves the cost of making a mistake

You can get that good to do it in your head. To get that good takes a hell of a lot of writing it out.

What he said. When starting out it is best to write it all down in binary so you understand what is going on. Grab a piece of paper and write EVERYTHING down, bit by bit. The question and all. That way you can visualize it in your head and helps to make it stick in your brain. After a while you will get to a point where you can just look at a number and work it out in your head. But to get to that point, as has already been said, it takes a LOT of writing everything down. Same thing with subnetting, which you should already be able to do in your head before doing route summarization/supernetting.

thedrama

control wrote: »

Hi All,

When you are summerizing routes - do you write it all out in binary as per all the articles I've read state or does there come a point where you can do it in your head?

if i recall correctly, you count the bits from left to right in order to supernet. For instance if you have /20, you count 20 bits beginning from left
until end of your subnet.

thedrama

Ltat42a wrote: »

Found this quick method, seems to work pretty good. Here's an example -

10101100.00010000 = 172.16
10101100.00010001 = 172.17
10101100.00010010 = 172.18
10101100.00010011 = 172.19
10101100.00010100 = 172.20
10101100.00010101 = 172.21
10101100.00010110 = 172.22
10101100.00010111 = 172.23
10101100.00011000 = 172.24
10101100.00011001 = 172.25
10101100.00011010 = 172.26
10101100.00011011 = 172.27
10101100.00011100 = 172.28
10101100.00011101 = 172.29
10101100.00011110 = 172.30
10101100.00011111 = 172.31

1. How many subnets are in the range? The RFC1918 Class B range is 16 subnets.

2. What power of 2 equals our range? 16 subnets = 2(4) so the answer is four.

3. Subtract the figure from step 2 from the default mask of our address range. In this example our default mask is 16 so the mask after subtracting 4 is /12.

4. Add this mask to the first address in the range - 172.16.0.0/12 in this example

It is nice you wrote all bits, though calculating in case of supernetting might be easier like this below :

You have got first 16 bits from left, right? Now, you have to find how many bits are 'same' among all beginning from left of the each subnet
until the end.

As you can see in the example, 10101100.0001

first 12 bits. So the answer is 172.16.0.0/12

10101100.00010000 = 172.16
10101100.00010001 = 172.17
10101100.00010010 = 172.18
10101100.00010011 = 172.19
10101100.00010100 = 172.20
10101100.00010101 = 172.21
10101100.00010110 = 172.22
10101100.00010111 = 172.23
10101100.00011000 = 172.24
10101100.00011001 = 172.25
10101100.00011010 = 172.26
10101100.00011011 = 172.27
10101100.00011100 = 172.28
10101100.00011101 = 172.29
10101100.00011110 = 172.30
10101100.00011111 = 172.31

CodeBlox

I can supernet in my head. I think it's just as easy as subnetting. All you do is make sure all of the subnets fall in range of your supernet and you've got a summary.