Subnet Design Exercise

 In 200-301 V1 BBB Not in new books, CCENT-OLD, IPv4 Design Drills

For the US holiday weekend, I thought I’d leave you with a design exercise that requires a little more thought than most of my exercise posts. In this case, you’ll start with a class B network, and a set of requirements. Your job: to come up with an IP subnetting design. You choose the mask, find all the subnets, choose the subnets, pick IP addresses from each subnet, and create the ip address commands on the routers. Answer after the fireworks next Monday. Enjoy, and for you US folks, happy Independence day!

You need to design and pick subnets for the following small internetwork:

The figure shows the required number of hosts that need to be supported. Your design should meet the following requirements:

  1. Choose 1 mask, and only one mask, to be used for all subnets
  2. Choose the 1 mask that meets the requirements while using the least number of host bits
  3. Assign the WAN subnets as the numerically lowest subnets from the list of possible subnets; the R1-R2 link with the lowest, and the R2-R3 link with the next lowest.
  4. Assign the LAN subnets with the three numerically highest subnet IDs; R1’s LAN with the highest, R2’s LAN next, and then R3’s LAN.
  5. Choose IP addresses for the router as the highest IP addresses in each subnet. On the WAN links, which have mlutiple router IP addresses in one subnet, give the highest IP address to the router with the higher router number (eg, R2 gets a higher IP address than R1)

I believe those requirements paint you into a corner well enough so that there is only one solution; if not, and my solution is different than yours, but you think yours is also valid, feel free to post. Also, post any clarifying questions you like before I post the answer.

Prep tools: Explain it back to someone
Answer (Part 1): Subnet Design Exercise
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[…] Jul Today’s post shows the beginning of a solution to the subnet design exercise I posted last week. This post isn’t all that meaningful without reading the other one first […]

[…] post picks up the discussion of the solution to the subnet design exercise I posted a while back. Start with the post for the original problem statement, and read from […]

Eric Ferland

ICND1 3ed edition “Analyzing Classful ipv4 Networks”Ch. 13 Page 376 2nd paragraph. “” should be a class C network not class B. I hope?


Hi Eric,
Yep, it’s a typo. It was meant to start with – just see the earlier reference to as the network. (It’s also listed in the errata page for the book here – Thanks, and sorry about the error…


Hi wendell odom in chapter 21 we have a term “broadcast subnet” i got your explanation point about it but im curious to know if for example :

we send a packet with destination address to broadcast subnet (last subnet i mean) what happens??

1.the packet will route to all subnet IDs in the network ?

2.or the packet will routed to that particular subnet ?

i think the second one is true but i want to know your feedback too .

tnx for your answer


I’ll ask a question of you, and then guess an answer for you as well!

First: you wrote “send a packet with destination address to broadcast subnet”. The broadcast subnet is an idea. An instance of a broadcast subnet has many numbers, including that subnet’s subnet ID, that subnet’s subnet broadcast address, and a range of usable addresses – just like any other subnet. The packet you describe would need to have one specific address. Which one?

That said, if I guess your meaning, if you sent a packet to some IP address in the range of usable addresses in some broadcast subnet, then yes, your thinking is correct. The routers will just route the packet based on their routes to that broadcast subnet.
Hope this helps,


HI wendell

I never used to be confused with networking terms and logic like this . after i wrote that comment i just thought about maybe i forgot sth wrong in my question of course i figured it out the packet is destined to a particular address not subnet and then
i wanted to write another comment to say it was my bad 🙂

I think it is because of a lot practice and getting around subnetting maybe cause confusion on for new ccna students like me 🙂

it’s requires time to become pro on it on exam and confusion is natural ( specially subnetting ) .

a lot thanks wendell for your support :))))


for your answer question wendell :
as a routing logic the packet would be routed to a particular subnet and to particular address by router use it routing table to forward the packet

i just thought about an idea that is it possible to send a packet and that packet would send to all subnets for example in ip network and all hosts on those subnets receive that packet ??
is it ?? so on that case what is the dest address would be?

of course it’s not ….

sorry if my question was odd to you i try to better revise it next time …

Best regard!


wrong typo : it’s not possible to be :)))


I worked on it .

one way to do that is to calculate all range subnet IDs (and their broadcast addr ) on that ip network and send copies to all those subnet IDs ( to their broadcast )

all of this is for this question :
is another way ( maybe easier ) to complete this mission :))


It would be useful to actually know which network one should work with, sure any class B network works as the 2 first octets are of no real relevance to the task, but still.

Abdelghani Attatfa
And the rest easier

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