Question

Consider three machines M, N, and P with IP addresses 100.10.5.2, 100.10.5.5, and 100.10.5.6 respectively. The subnet mask is set to 255.255.255.252 for all the three machines. Which one of the following is true?

• A. M, N, and P all belong to the same subnet
• B. Only M and N belong to the same subnet
• C. Only N and P belong to the same subnet
• D. M, N, and P belong to three different subnets

Hint:

For a {/30} subnet, IP addresses are grouped in blocks of 4. If two IPs differ by less than 4 in the last octet and fall in the same block, they belong to the same subnet.

Answer 1

The Correct Option is C

Step 1: Understand the subnet mask.
The given subnet mask is $255.255.255.252$, which corresponds to a /30 network.
For a /30 subnet:
Block size $= 256 - 252 = 4$
Total IP addresses per subnet $= 4$
Usable host addresses per subnet $= 2$
Step 2: Determine the subnet ranges.
With a block size of 4, the subnet ranges in the last octet are:
$0\!-\!3,\; 4\!-\!7,\; 8\!-\!11,\; \dots$
Step 3: Find the subnet of each machine.
Machine M:
IP address = 100.10.5.2
This lies in the range $0\!-\!3$.
Subnet = 100.10.5.0/30
Machine N:
IP address = 100.10.5.5
This lies in the range $4\!-\!7$.
Subnet = 100.10.5.4/30
Machine P:
IP address = 100.10.5.6
This lies in the range $4\!-\!7$.
Subnet = 100.10.5.4/30
Step 4: Compare the subnets.
Machines N and P belong to the same subnet, while machine M belongs to a different subnet.
Step 5: Conclusion.
Only machines N and P belong to the same subnet.