Some things never change. CCENT and CCNA candidates still have the roughest time in the curriculum with the topic of subnetting.

Hey! No problem! We have all been there. Just remain patient, remain calm, and keep working through examples and practice problems.

Do you want a quick quiz to see if your skills are up to speed? Check out this blog post:

Subnetting Practice Quiz 1

Let’s walkthrough a common subnetting question type in this blog entry. Here is the question, followed by how I would solve it in the written exam on my scratch paper.

I immediately think about the analogy in the CCENT course about street address and house number here. They are asking for the street address (network portion) of this address. The host ID is my house number.

Well, the contiguous bits in the mask identify the network portion of the IP address. I can see from the 255.255.255 portion of the mask that my street address definitely begins as follows:

172.16.129.?

The real question here is what value is in the forth octet?

To solve this, I create my “cheat sheet” conversion table on the scratch paper:

2^7  2^6  2^5    2^4     2^3     2^2    2^1     2^0
128   64    32       16         8        4           2          1

Converting 180 to decimal and 128 to binary produces the following:

IP Address – Forth Octet:        10110100
Subnet Mask – Forth Octet:   10000000

When you AND (multiply) each IP address bit position against the subnet mask, you end up with the network identifier. Here the result is simple – 10000000

172.16.129.128

Let’s have you try one!

### 9 Responses to “What Street Do You Live On?”

1. Try taking the octet in question and subtracting it from 256. I find this method by far the most straight forward and easiest to do in your head.

For example with a subnet mask of 255.255.255.224 you would do 256 – 224 which equals 32. For that subnet mask gives you 32 IPs.

2. Peter says:

This is how I do it:

The mask is 255.255.224.0. The first number that is not 255 tells us where to count the subnets and by how much. In this case it is 224 which is in the third octet of the mask. Remember these pieces for later.

Take the number and subtract it from 256 to find out what to count by. In this case 256-224 = 32, so count by 32 in the third octet. 0, 32, 64, 96, 128, etc.

The third octet of the given IP address fits between 96 and 128 so 96 in the third octet is the start of subnet. This becomes 10.12.96.0 when you write it out fully.

3. really nice explanation thanks for that

4. David Cruz says:

Here is a slight improvement to Peter’s answer. Instead of subtracting 224 from 256 to find the subnet block size, simply determine the value of the last bit in the mask. In this case it is 32.

2^7 2^6 2^5
128 64 32

5. Kaushikkumar says:

Mine is in any broken octet as below x 255 full octet -2.

===
Mask:128 192 224 240 248 252 254 255
Host:128 64 32 16 8 4 2 1

6. [...] See the original post here: What Street Do You Live On? [...]

7. vicky says:

8. Matthew says:

Take 256 (Possible IP addresses per Octet)and minus the third octet of the broadcast address (224) to find the number range of each subnet which gives you 32 (256-224 = 32)

Count up by 32 until you get to or close to the third octet number of the IP address (100)
0
32
64
96
128

100 lies in between 96 and 128 so it belongs to the range of 96-127.