CCNA FAQ: Variable Length Subnet Masks
Q1. Which of the following routing protocols support VLSM?
Answer: B, C, and D
Q2. What does the acronym VLSM stand for?
a. Variable length subnet mask
b. Very long subnet mask
c. Vociferous longitudinal subnet mask
d. Vector-length subnet mask
e. Vector loop subnet mask
Explanation: A. Note that sometimes VLSM stands for variable-length subnet masking, which refers to the process of using different masks in the same classful network, whereas variable length subnet mask refers to the subnet masks themselves.
Q3. R1 has configured interface Fa0/0 with the ip address 10.5.48.1 255.255.240.0 command. Which of the following subnets, when configured on another interface on R1, would not be considered an overlapping VLSM subnet?
a. 10.5.0.0 255.255.240.0
b. 10.4.0.0 255.254.0.0
c. 10.5.32.0 255.255.224.0
d. 10.5.0.0 255.255.128.0
Explanation: A. Subnet 10.5.0.0 255.255.240.0 implies range 10.5.0.0 – 10.5.15.255, which does not overlap. 10.4.0.0 255.254.0.0 implies range 10.4.0.0 – 10.5.255.255, which does overlap. 10.5.32.0 255.255.224.0 implies range 10.5.32.0 – 10.5.63.255, which does overlap. 10.5.0.0 255.255.128.0 implies range 10.5.0.0 – 10.5.127.255, which does overlap.
Q4. R4 has a connected route for 172.16.8.0/22. Which of the following answers lists a subnet that overlaps with this subnet?
Explanation: D. The four answers imply the following ranges: 172.16.0.0/21: 172.16.0.0 – 172.16.7.255. 172.16.6.0/23: 172.16.6.0 – 172.16.7.255. 172.16.16.0/20: 172.16.16.0 – 172.16.31.255. 172.16.11.0/25: 172.16.11.0 – 172.16.11.255. The subnet in the question, 172.16.8.0/22, implies a range of 172.16.8.0 – 172.16.11.255, which includes the range of numbers in subnet 172.16.11.0/25. ptg6876495 674 Appendix A: Answers to the “Do I Know This Already?” Quizzes
Q5. A design already includes subnets 192.168.1.0/26, 192.168.1.128/30, and 192.168.1.160/29. Which of the following subnets is the numerically lowest subnet ID that could be added to the design, if you wanted to add a subnet that uses a /28 mask?
Explanation: C. The question lists three existing subnets, which together consume part of class C network 192.168.1.0. Just listing the last octet values, these subnets consume: 0 – 63, 128 – 131, and 160 – 167. The new subnet, with a /28 mask, needs 16 consecutive numbers, and the subnet numbers will all be a multiple of 16 in the last octet (0, 16, 32, etc.). Looking at the consumed numbers again, the first opening starts at 64, and runs up through 127, so it has more than enough space for 16 addresses. So the numerically lowest subnet number is 192.168.1.64/28, with range 192.168.1.64 – 192.168.1.79.
Q6. An engineer is following a VLSM design process of allocating the largest subnets first, as the numerically lowest subnets, and then subdividing the next subnet into smaller pieces for the next smaller size of subnet. In this case, the engineer has reserved the first three /20 subnets of 172.16.0.0 to be used in an internetwork: 172.16.0.0/20, 172.16.16.0/20, and 172.16.32.0/20. The next smaller size subnets to be allocated will be subnets with mask /25; this design requires 10 such subnets. Assuming the engineer continues to allocate subnets in sequence, which answers lists the tenth of these /25 subnets?
Explanation: C. The next /20 subnet in sequence would have been 172.16.48.0/20, so 172.16.48.0 is the lowest number that could be used for the next subnet ID. To find the next 10 subnets in sequence, using a /25 mask, start with 172.16.48.0, and find all the /25 subnets. Those include 172.16.48.0, 172.16.48.128, 172.16.49.0, 172.16.49.128, and so on, up through the 10th such subnet, 172.16.52.128.