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: Decode the given subnet mask. 
The subnet mask provided is $255.255.255.252$, which represents a /30 network.
In a /30 subnet:

• The address block size is $256 - 252 = 4$
• Each subnet contains 4 IP addresses
• Out of these, only 2 addresses can be assigned to hosts

Step 2: List the subnet intervals.
Since the block size is 4, the subnets in the last octet progress as:

\[ 0\!-\!3,\; 4\!-\!7,\; 8\!-\!11,\; \ldots \]

Step 3: Identify the subnet for each machine.

Machine M:
IP address: 100.10.5.2
This address falls within the $0\!-\!3$ range.
Therefore, it belongs to the subnet:

\[ 100.10.5.0/30 \]

Machine N:
IP address: 100.10.5.5
This address lies in the $4\!-\!7$ range.
Hence, its subnet is:

\[ 100.10.5.4/30 \]

Machine P:
IP address: 100.10.5.6
This address also falls in the $4\!-\!7$ range.
So, it belongs to the subnet:

\[ 100.10.5.4/30 \]

Step 4: Compare subnet membership.
Machines N and P share the same subnet, whereas machine M is part of a different subnet.

Step 5: Final conclusion.
Only machines N and P are on the same subnet.