Exam-style question
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Explain how a subnet mask is used to find the network identifier from an IP address. Use the IP address 10.20.35.14 and the subnet mask 255.255.255.0 in your answer.
Model answer
What a good answer should say
- The subnet mask is compared with the IP address one bit at a time.
- A mask value of 255 means that all eight bits in that octet are part of the network identifier, while a mask value of 0 means that the octet is not part of the network identifier.
- Therefore, for 10.20.35.14 with the mask 255.255.255.0, the first three octets form the network identifier and the final octet is set to zero.
- The network identifier is 10.20.35.0.
Explanation
Why this works
The mask marks the network portion of the address. Applying the mask to the address, commonly described as a bitwise AND, preserves the first three octets and clears the final octet.
Common mistake
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