Chapter 22 - Variable-Length Subnet Masks

Question 1

Which of the following routing protocols support VLSM? (Choose three answers.)

a. RIPv1
b. RIPv2
c. EIGRP
d. OSPF

Answer 1

B, C, and D.

Classless routing protocols, by definition, support VLSM because classless routing protocols transmit the subnet mask in their routing updates. Of the listed answers, only RIP version 1 (RIPv1) is not a classless routing protocol.

Question 2

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

Answer 2

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.

Question 3

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

Answer 3

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.

Question 4

R4 has a connected route for 172.16.8.0/22. Which of the following answers lists a subnet that overlaps with this subnet?

a. 172.16.0.0/21
b. 172.16.6.0/23
c. 172.16.16.0/20
d. 172.16.11.0/25

Answer 4

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.
173. 16.16.0/20: 172.16.16.0 – 172.16.31.255.
174. 16.11.0/25: 172.16.11.0 – 172.16.11.127.

The subnet in the question, 172.16.8.0/22, implies a range of 172.16.8.0 – 172.16.11.127, which includes the range of numbers in subnet 172.16.11.0/25.

Question 5

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?

a. 192.168.1.144/28
b. 192.168.1.112/28
c. 192.168.1.64/28
d. 192.168.1.80/28
e. 192.168.1.96/28

Answer 5

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, and so on). 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.