Mask Design Drill 1: Answers
Today’s post just lists the answers to the Mask Design drill #1. The drill lists some basic requirements for a number of hosts per subnet, and a number of subnets, and a network class. Your job: Pick the one mask that meets all the requirements. Today’s post lists the answers. My ICND1 book, Chapter 16 spells out the details. Post questions here if you have them!
A Quick Note on What’s in the Answers
The Drill asks for the one and only one prefix-style mask that meets all requirements, or “none” if no mask meets all requirements. This post lists an answer for each problem in an upcoming answer table. It also lists a few other key facts that are interim steps, plus a diagram that shows the main concepts behind the choices of masks.
I’m leaving the explanations at that; ask questions if the concept doesn’t leap at you after seeing what’s here.
Mask Design, Drill 1: Answers
The last two columns list the literal answers as requested in the original drill post.
Table 3: Mask Design Problems: Answers
| Class | Need at Least This Many Subnet Bits | Need at Least This Many Host Bits | Range of Masks | Correct Prefix Mask | |
| 1 | A | 8 | 9 | /16 – /23 | /23 |
| 2 | B | 8 | 7 | /24 – /25 | /24 |
| 3 | C | 4 | 4 | /28 | /28 |
| 4 | B | 6 | 11 | None | None |
Figure 1: Concepts Behind Drill 1, Problem 1
Figure 2: Concepts Behind Drill 1, Problem 2
Figure 3: Concepts Behind Drill 1, Problem 3
Figure 4: Concepts Behind Drill 1, Problem 4
