May
24

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.


"You run the ipconfig command and discover your IP address and subnet mask are: 172.16.129.180/255.255.255.128. What is your network address?"

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

Our street address is:

172.16.129.128

Let's have you try one!


"You run the ipconfig command and discover your IP address and subnet mask are: 10.12.100.20/255.255.224.0. What is your network address?"

Have fun working though it. Post your solution, and your solution approach, in the comments below.

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