# Subnet Design Exercise 3

The subnet design exercises require that you do some subnetting math, but rather than reacting like you would in a help desk or support job role, you plan as if you were in the role of a lead network engineer. If you have seen the other subnet design exercises, you already know the drill. If not, read below the fold. The short version: you get some exercise in choosing the best subnet mask based on requirements, and then in finding all subnets, and choosing the best subnets to use, again based on a list of requirements. Answer to follow in a later post. Here are some helpful links:

## Problem Statement

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

#### Figure 1: Network Diagram and Requirements

Note that the figure lists the requirements for the number of hosts per subnet, plus the fact that you must plan for 20% growth in the numbers of hosts per subnet on the LANs.  Additionally, follow these requirements as well:

1. The LAN subnets should come from class B network 172.24.0.0.
2. The WAN subnet should also come from class B network 172.24.0.0.
3. Choose 1 mask per classful network, and only one mask, to be used for all subnets
4. For each classful network, choose the 1 mask that meets the requirements while using the least number of host bits
5. 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.
6. 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.
7. Choose IP addresses for the router as the highest IP addresses in each subnet.
8. 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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