## Subnet Design Exercise 2

Today’s exercise requires a little more thought than most of my exercise posts. In this case, you’ll start with two class B networks: 172.20.0.0 and 172.30.0.0. The problem also lists a set of requirements. Your job: to come up with an IP subnetting design. You choose the one mask to use in each network, find all the subnets, and choose the subnets. Answer to follow. Enjoy!

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

Your design should meet the following requirements:

1. The LAN subnets should come from class B network 172.20.0.0, and the WAN subnets should come from class B network 172.30.0.0.
2. Choose 1 mask per network, and only one mask, to be used for all subnets in each of the two networks
3. For each network, choose the 1 mask that meets the requirements while using the least number of host bits
4. Assign the WAN subnets as follows: give the R1-R2 links the numerically lowest subnet ID, the R2-R3 link the next lowest, and the R1-R3 link the next lowest.
5. 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.
6. Choose IP addresses for the router as the highest IP addresses in each subnet.
7. 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.

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R1 – R2 172.30.0.0/30 R1 – .1 / R2 – .2
R2 – R3 172.30.0.4/30 R2 – .5 / R3 – .6
R3 – R1 172.30.0.8/30 R1 – .9 / R3 – .10

LANS
R1 LAN – 172.20.255.0/24 FA0/0 172.20.255.254
R2 LAN – 172.20.254.0/24 FA0/0 172.20.254.254
R3 LAN – 172.20.253.0/24 FA0/0 172.20.253.254

Hey Dan,
Thanks… FYI, I usually let it percolate a few days before I comment – others may, I’ll just wait to give my own opinion to give time for folks to see/work. Just replied to your other post as well.
Wendell

[…] post shows the solution to subnet design exercise 2, specifically the IP subnets. This post isn’t all that meaningful without reading the other […]

Hi Wendell – You might ought to fix this part too…

“Today’s exercise requires a little more thought than most of my exercise posts. In this case, you’ll start with two class B networks: 172.16.20.0 and 172.16.30.0. ”

See above…May save folks some time.. I am redoing my lab now 🙂

Hi Jon,