# Subnetting - Reverse Engineering

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Good Day,

I know this may have been asked before but I am having a bit of problems using the search function on my phone ... (so I apologize in advance if this is the case)

I am having a small issue with reverse engineering subnets which from what I can tell is the most popular type of question for the CCNA exam ...

I understand the basics to subnetting (creating a certain number of subnets or making subnets based on the hosts needed, etc) but with reverse engineering I am just plain slow.

When the increment is larger it is fine as the number of subnets is less but when when the increment is 16 or less and the question asks which subnet the host is on, or what is the first valid IP on the subnet that the host is on, I struggle.

For example :

Which Subnet does host : 172.27.25.156 255.255.255.240

I know that 240 is /28 (if I am correct) so the increment is 16 ... ... Means I need to write out each subnet which is 9 subnets to get the answer ... Which is 172.27.25.144

Is the simplest way really to learn my 16 times table and just write out the subnets or is there a quicker way ?

Regards,
Richard.

• Member Posts: 365 ■■■■□□□□□□
If I know the increment is 16/8/4, I find the closest number I know is valid and work from there. Like in your example, I know the increment is 16 and that 16 * 10 = 160. All I'd do is subtract 16 from 160 to get 144.

Or, if I were trying to find the subnet for 172.16.25.215/29, I might jump to 224 since I know it's a multiple of 32 (which means it must be a multiple of 8 ), and subtract 16 (or as I usually picture it in my head, 8 twice) to get 208.
• Member Posts: 57 ■■□□□□□□□□
I use something called 'anding' which I learned from Wendell Odom's book. You take the IP address in question and the subnet mask and put one on top of the other, convert them both to binary.

255.255.255.240 = 11111111.11111111.11111111.11110000
172.27.25.156 = 10101100.00011011.00011001.10010000
10101100.00011011.00011001.10010000

There will be 32 columns - one for each bit. Each column that has two ones gets a 1, every other column gets a 0. When you get your result, it will give you the network ID (subnet).

I can't get the columns to line up right in my post, but you get the idea right?
• Member Posts: 587 ■■■□□□□□□□
I made this handy chart in Excel, it works pretty good.
• Member Posts: 57 ■■□□□□□□□□
Ltat42a wrote: »
I made this handy chart in Excel, it works pretty good.

Nice job!
• Member Posts: 191
I have said this before on here, but every time I took a shower for a couple of months I sung my 16 times table out loud, this has been proved to be very useful. Even when I now say them out loud in a room of engineers they are impressed.
• Member Posts: 23 ■□□□□□□□□□
Anding makes life so much easier !! ... thank guys !!!! ...

How long are the CCNA exams ?? ... ... What is a good time to work towards for each type of subnetting ?
• Member Posts: 272
I do something similar to what boredgamelad does. Rather than start from 0 and work up, I start from a number I know is a multiple of it and work up. 32, 64, 96, 128, 192, etc. If it's a multiple of 32 then it's definitely a multiple of 8, or 4, or whatever.
Current objective: CCNA Security
My blog: mybraindump.co.uk
• Member Posts: 57 ■■□□□□□□□□
iamme4eva wrote: »
I do something similar to what boredgamelad does. Rather than start from 0 and work up, I start from a number I know is a multiple of it and work up. 32, 64, 96, 128, 192, etc. If it's a multiple of 32 then it's definitely a multiple of 8, or 4, or whatever.

I've seen multiple ways of doing it. Basically it all comes down to what works best for each person. For the most part, I can just do it in my head these days. I practiced subnetting like a madman.