Which two VLSM subnets, when taken as a pair, overlap? (Choose two.)

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Variable Length Subnet Masking (VLSM) allows the creation of subnets with different sizes, which can be very useful when designing a network. However, if VLSM is not properly planned, overlapping subnets can occur, causing communication issues.

To determine which subnets overlap, we need to look at their network addresses and subnet masks. The network address is the IP address with all the host bits set to zero. The subnet mask determines the size of the subnet by specifying how many bits are used for the network portion of the IP address.

Let's analyze each subnet one by one:

A. 10.22.21.128/26

This subnet has a network address of 10.22.21.128 and a subnet mask of 255.255.255.192. The subnet mask indicates that 26 bits are used for the network portion, leaving 6 bits for the host portion. The range of addresses in this subnet is 10.22.21.128 - 10.22.21.191.

B. 10.22.22.128/26

This subnet has a network address of 10.22.22.128 and a subnet mask of 255.255.255.192. The subnet mask indicates that 26 bits are used for the network portion, leaving 6 bits for the host portion. The range of addresses in this subnet is 10.22.22.128 - 10.22.22.191.

C. 10.22.22.0/27

This subnet has a network address of 10.22.22.0 and a subnet mask of 255.255.255.224. The subnet mask indicates that 27 bits are used for the network portion, leaving 5 bits for the host portion. The range of addresses in this subnet is 10.22.22.0 - 10.22.22.31.

D. 10.22.20.0/23

This subnet has a network address of 10.22.20.0 and a subnet mask of 255.255.254.0. The subnet mask indicates that 23 bits are used for the network portion, leaving 9 bits for the host portion. The range of addresses in this subnet is 10.22.20.0 - 10.22.21.255.

E. 10.22.16.0/22

This subnet has a network address of 10.22.16.0 and a subnet mask of 255.255.252.0. The subnet mask indicates that 22 bits are used for the network portion, leaving 10 bits for the host portion. The range of addresses in this subnet is 10.22.16.0 - 10.22.19.255.

To determine which subnets overlap, we need to compare their network addresses and subnet masks. Subnets A and B have the same subnet mask and overlap in the range of addresses from 10.22.22.128 to 10.22.21.191. Subnets D and E do not overlap with any of the other subnets. Subnet C overlaps with subnet B in the range of addresses from 10.22.22.0 to 10.22.22.31.

Therefore, the two VLSM subnets that overlap are subnets A and B, and subnets B and C.