Subnet Speed Practice #2: Answers

 In 200-301 V1 Ch14: Analyzing IPv4 Subnets, 200-301 V1 Part 4: IPv4 Addressing, CCENT-OLD, IPv4 Address Drills

This post makes no sense without reading this post first. The earlier post lists 5 subnetting problems, and tells you to time yourself. The answers are below the fold in this post. Don’t look til you try it for yourself! Post questions if you have them.

Problem Network Bits Subnet Bits Host Bits # Hosts
10.1.1.1/23 8 15 9 510
172.16.203.203/25 16 9 7 126
192.168.1.161/29 24 5 3 6
10.1.99.101/26 8 18 6 62
172.16.77.177/27 16 11 5 30
  Subnet ID 1st Addr. last Addr. B’cast
1 10.1.0.0 10.1.0.1 10.1.1.254 10.1.1.255
2 172.16.203.128 172.16.203.129 172.16.203.254 172.16.203.255
3 192.168.1.160 192.168.1.161 192.168.1.166 192.168.1.167
4 10.1.99.64 10.1.99.65 10.1.99.126 10.1.99.127
5 172.16.77.160 172.16.77.161 172.16.77.190 172.16.77.191
Subnetting Speed Practice 2
Serial Link Interview Question
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Is 2min per question too slow?

JAMES

Took me 12 min – 30 sec; practice makes perfect!
Will keep at it!

lyjo

James – that is the right attitude!
Helps to do a little every day! Do it!

Have fun, hope you’re sticking with it!
Wendell

Bryant

It took me 8 mins. I am going to keep practicing

parvesh

192.168.1.161/29
is broadcast address right
isn’t it should be 192.168.1.163

Chris

Hello Parvesh

/29 – 255.255.255.248
256 – 248 = 8
192.168.1.160 – Network address
192.168.1.161 – 1st host
192.168.1.166 – last host
192.168.1.167 – Broadcast

Jorge

Huff, took me 14:18:81

Wendell Odom

Hey – you will improve! Like getting back into shape! You can do it!

Albert

I think the answer for the first address for sub 10.1.0.0/23 is wrong. It must be 10.1.1.0 but you put 10.1.0.1

Are you agree with me?

Wendell Odom

Hi Albert,
Thanks for the post! I disagree, but that’s what these exercises are great for – give it a try, struggle a bit, and learn.
Thinking it decimal, whatever the subnet ID is, add 1 to the 4th octet to get the numerically lowest address.
Conceptually, I think you’re thinking about the /23 mask as creating 1 host bit in the 3rd octet, but ignoring the 8 host bits in the 4th octet.
The usable addresses in the subnet are 10.1.0.1, 10.1.0.2, .3, .4, etc to 10.1.0.255, then 10.1.1.0, 10.1.1.1, 10.1.1.2, .3, etc to 10.1.1.254 (last usable), the 10.1.1.255 as the subnet broadcast address.

Hope this helps.
Wendell

Albert

Dear Wendell,

I just noticed my logic was totally upside down, so yes your explanation helps.

Now it’s burned in my memory : ” Add 1 to the 4th octet whatever there is before to get the first address”.

Thank you for help.

Wendell Odom

You’re quite welcome, Albert.

Dayne

I am confused on whether we’re using classless or classful addressing.

As I understand it, if we were using classless IPv4 logic, the “network” and “subnet” portion would be combined into a network “prefix” consisting of 23 bits. If we wanted to create additional subnets, we would “borrow” from the host bits.

Using classless logic, I arrived at the answers provided.

This is where I’m confused: if we used classful IPv4 logic, wouldn’t the subnet ID of the first subnet be 10.0.0.0 for question 1? We would “borrow” 15 host bits (i.e. create 15 “subnet” bits) to divide our network into 32,768 subnets with 510 hosts per subnet.

I apologize for any confusion. My test is about a month away, and my brain is a little cooked from slamming through this material. Thank you for taking time out of your day to help us would-be network engineers. Cheers!

Wendell Odom

Hi Dayne,
The terms “classless addressing” and “classful addressing” confuse a lot of folks. It’s not that uncommon to find documents that differ in what they claim the terms to mean. I’ve been attempting to be consistent and clear about these terms in my books for 25+ years now, so if you have those, I encourage you to look at Chapter 13’s “Classless and Classful Addressing” section.
That said… the terms refer to a way to think. They do not change or impact how any devices behave. They are different views for us humans when we think about different aspects of IP addressing and subnetting.
Any time you think about Subnetting a class A/B/C network, you must think with classful addressing logic. That logic gives you a three-part view of the address structure: Network, Subnet, and Host.
On the other hand, when you are thinking about the structure as two parts, you are using classless logic. One part is the host part. The other is the prefix part, which is the part that has the same value for all addresses in a subnet. For example, when thinking about one specific subnet, you can think with classless addressing logic, as the division between network and subnet does not matter. When looking at the IP routing table on a router, you can think with classless addressing, thinking about which packets match each prefix (aka subnet.)

So, to your comments. Your paragraph that begins “This is where I’m confused…” correctly describes thoughts that use classful addressing logic. It describes a network. Unsubnetted, the structure has 8 network and 24 host bits. You then borrowed 15 host bits to create subnet bits. So you’re thinking about network, subnet, and host parts. That’s classful addressing.

Your paragraph beginning “As I understand it…” – I agree with the first sentence and it describes classless addressing. But I see the second sentence as either unrelated, or a thought that goes with classful addressing. That is, if you’re thinking about “borrowing bits”, then that is jargon associated with the subnet design process, starting with a classful network, which unsubnetting has a network and host part, and borrowing some of the host bits to create a subnet part. Three parts, so classful addressing logic.

Finally… I wouldn’t worry about the differences in meanings between the two terms. Honestly, understanding subnetting concepts, how to do the math, and so on is 100,000X more important than putting a label on whether you were using classless or classful addressing concepts. But since you asked, that dividing line is simply this: If you need to distinguish between the network and subnet parts, it’s classful addressing, and if you can ignore that distinction (and treat network+subnet as a prefix), then it’s classless addressing.

Hope your final month of study goes well. Keep at it!
Wendell

Dayne

That clears a lot up, thank you!

Solar winds has a graphical subnetting calculator that conceptualized IPv4 addresses as having “mask bits,” “host bits” and “subnetting bits.” Before reading your book, I used it to develop my understanding of subnetting. It used classless logic; so, the “mask bits” served the same function as the network prefix.

My confusion stems from the calculator conceptually… double-dipping (for a lack of a better way to put it) and framing subnetting bits as something you can create in addition to a network prefix. As you said, there are many ways for humans to conceptualize these concepts. I’ll keep doing my best to grok these ideas.

Also, I totally misread the question. I now understand that the goal is to find the requested subnet info corresponding to a particular IP addresses and mask, NOT to to find the first subnet-ID, first subnet broadcast address and etc. “I see,” said the blind person haha.

Thanks again for your help! Have yourself a blessed weekend!

Wendell Odom

Dayne,
Glad to be of help!
Lots of little pieces have to click into place with subnetting… good stuff!

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