Exam Power Tips – Subnetting Class C for Network+ N10-006

Exam Power Tips – Subnetting Class C for Network+ N10-006

By Aaron Axline, A+, Server+, and MCP. Industry consultant and author and/or editor for Exam Cram, Que Publishing, and Certification Magazine. Aaron’s blog.

 

Of all of the least-uttered sentences in the history of spoken language, right at the top is: “I just love to manually subnet TCP/IP networks!” But in the exam room at a test center, you need to know how to manually solve problems like this:

Given the Class C network address 192.168.10.0 and a subnet mask of 255.255.255.248…

…How many subnets can be created?

…What is the address range of the first subnet?

…How many clients can be hosted on each subnet?

In this article, we’re going to show you power tips for working through Class C network subnetting exam questions. These power tips use shortcuts that you write down in the exam room before starting the exam.

We are skipping past mathematical formulas and binary conversions here. Don’t worry: in real life you’ll use a subnetting calculator. Exams aren’t real life.

REMINDER: Class C IP addresses have a first octet value between 192-223 and a default subnet mask of 255.255.255.0.

Know the six subnet masks

First thing: the second you sit down in the exam room with your test center-provided marker board or scrap paper, write down these six subnet masks:

255.255.255.128
255.255.255.192
255.255.255.224
255.255.255.240
255.255.255.248
255.255.255.252

These are the only subnet masks you need to worry about for Class C network subnetting. All of the power tips that follow are based on this list.

Subnets and hosts

Next, write down the total number of subnets for each subnet mask. To do this, start from the top of the subnet mask list, begin with 2 subnets, and double the number for each consecutive subnet mask, like so:

255.255.255.128 = 2 subnets
255.255.255.192 = 4 subnets
255.255.255.224 = 8 subnets
255.255.255.240 = 16 subnets
255.255.255.248 = 32 subnets
255.255.255.252 = 64 subnets

Next, write down the maximum number of hosts per subnet. This time, start from the bottom of the subnet mask list and work up. Begin with 2 hosts, then double the number and add 2 to it for each consecutive subnet mask, like so:

255.255.255.128 = 126 hosts/subnet (62 doubled +2)
255.255.255.192 = 62 hosts/subnet (30 doubled +2)
255.255.255.224 = 30 hosts/subnet (14 doubled +2)
255.255.255.240 = 14 hosts/subnet (6 doubled +2)
255.255.255.248 = 6 hosts/subnet (2 doubled + 2)
255.255.255.252 = 2 hosts/subnet

Now your scrap paper or marker board should look something like this:

255.255.255.128 = 2 subnets = 126 hosts/subnet
255.255.255.192 = 4 subnets = 62 hosts/subnet
255.255.255.224 = 8 subnets = 30 hosts/subnet
255.255.255.240 = 16 subnets = 14 hosts/subnet
255.255.255.248 = 32 subnets = 6 hosts/subnet
255.255.255.252 = 64 subnets = 2 hosts/subnet

Next, multiply across each row to get the total number of usable hosts for each subnet mask.

255.255.255.128 = 2 subnets = 126 hosts per subnet = 252 total hosts
255.255.255.192 = 4 subnets = 62 hosts per subnet = 248 total hosts
255.255.255.224 = 8 subnets = 30 hosts per subnet = 240 total hosts
255.255.255.240 = 16 subnets = 14 hosts per subnet = 224 total hosts
255.255.255.248 = 32 subnets = 6 hosts per subnet = 192 total hosts
255.255.255.252 = 64 subnets = 2 hosts per subnet = 128 total hosts

SUPER Power Tip: Notice how the total number of usable hosts is the same as the last octet of the subnet masks, but in reverse order? You can use this shortcut to quickly jot down the total usable hosts instead of multiplying numbers.

Subnet ranges and reservations

Next, write down the subnet range or each subnet mask. To do this, start from the bottom of the subnet mask list, begin with a range of 4, and double each number as you go up.

255.255.255.128 = 128 subnet range
255.255.255.192 = 64 subnet range
255.255.255.224 = 32 subnet range
255.255.255.240 = 16 subnet range
255.255.255.248 = 8 subnet range
255.255.255.252 = 4 subnet range

Finally, write these two points below your notes:

The first IP address in a subnet range is reserved for the subnet ID.
The last IP address in a subnet range is reserved for broadcasts.

Voila! Power Notes!

Here is what your final notes should look like:

255.255.255.128 = 2 subnets = 126 hosts per subnet = 252 total hosts = 128 subnet range
255.255.255.192 = 4 subnets = 62 hosts per subnet = 248 total hosts = 64 subnet range
255.255.255.224 = 8 subnets = 30 hosts per subnet = 240 total hosts = 32 subnet range
255.255.255.240 = 16 subnets = 14 hosts per subnet = 224 total hosts = 16 subnet range
255.255.255.248 = 32 subnets = 6 hosts per subnet = 192 total hosts = 8 subnet range
255.255.255.252 = 64 subnets = 2 hosts per subnet = 128 total hosts = 4 subnet range

The first IP address in a subnet range is reserved for the subnet ID.
The last IP address in a subnet range is reserved for broadcasts.

With these notes, you should be able to answer any Class C subnetting question you encounter on the exam.

Example

Let’s go back to the sample questions from the start of this article.

You have an IP address of 192.168.10.0 and a subnet mask of 255.255.255.248…

…How many subnets can be created?

From your notes, you know the answer is 32.

…What is the address range of the first subnet?

The subnet range is 8; so the first subnet is 192.168.10.0 to 192.168.10.7. The second subnet begins with 192.168.10.8. The third subnet begins with 192.168.10.16, and so on.

…How many clients can be hosted on each subnet?

There are 8 IP addresses per subnet, but remember the first and the last addresses are reserved, so there can only be 6 hosts per subnet. In this case, the usable host IP addresses in the first subnet would be:

192.168.10.0 [reserved]

192.168.10.1
192.168.10.2
192.168.10.3
192.168.10.4
192.168.10.5
192.168.10.6

192.168.10.7 [reserved]

NOTE: Subnetting is found in the Network+ Exam N10-006 in Main Domain 1.0 “Network architecture” under sub-objective: 1.8 “Given a scenario, implement and configure the appropriate addressing schema.”

Subnetting is also found in the Security+ Exam SY0-401-006 in Main Domain 1.0 “Network security” under sub-objective: 1.3 “Explain network design elements and components.”

Click here for the CertBlaster Network+ N10-006 practice tests

Click here for the CertBlaster Security+ SY0-401 practice tests

5 thoughts on “Exam Power Tips – Subnetting Class C for Network+ N10-006

  1. Nice trick, but I have another one.

    Given the class C subnet mask, consider it as made of 4 dot-separated string of 8 bits, and notice how the last octet of bits is divided:

    In 192.168.10.0/255.255.255._248_
    the _248_ is made of 11111000

    1) NUMBER OF SUBNETS. You have 5 consecutive 1, and being them bits, you obtain the number of subnets as:

    2^5 = 2*2*2*2*2 = 32 subnets.

    2) NUMBER OF SUBNET RANGES. You have 3 consecutive 0 bits, so you have:

    2^3 = 2*2*2 = 8 subnet ranges.

    3) CLIENTS HOSTED PER SUBNET. It’s the number of subnet ranges, minus the first and last address which are reserved:

    8 subnet ranges -2 = 6 clients per subnet.

    4a) TOTAL HOSTS. It’s the number of subnet, multiplied * the clients hosted per subnet:

    32 subnets * 6 clients per subnet = 192 total hosts.

    4b) TOTAL HOSTS. Better yet, if you consider our string of bits as made of two separate binary numbers, all you have to do is complement “bit per per bit” the second number, shift it to the left of as many positions as the digits is made of, then subtract it from the first number.

    _248_ => 11111000, so a =11111 and b=000

    b=000 and has 3 digits => ~b=111 and you shift it 3 times, let’s call it c = 111000

    a – c = 11000000 which is the binary representation of 192.

    Voila’, no tables to write or remember, and once you do it a pair of times will become so natural you’ll do it effortlessly.

    1. Hi Rick, I am not sure what Dario’s method is. The one you saw in the post is Aaron’s, is that what you are asking about? If nopt send us a link to Dario’s and I am sure one of our experts can do this for you.

  2. Sorry about that, hopefully this is a bit clear.

    Is there an easy way to user Aaron’s method but for class B subnets? I’ve memorized the chart but can’t seem to figure out the answers in regards to 255.255.X ranges.

    Thanks

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