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Module 3: Network Layer & Routing

20202m

What is the default subnet mask for a class A network?

(i) 127.0.0.1
(ii) 255.0.0.0
(iii) 255.255.255.0
(iv) 255.255.255.0

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Answer

(ii) 255.0.0.0

Explanation

In the traditional classful IPv4 addressing scheme, a Class A network uses:

  • 8 bits for the network portion.
  • 24 bits for the host portion.

Therefore its default prefix is /8.

The corresponding subnet mask is:

255.0.0.0255.0.0.0

In binary:

11111111.00000000.00000000.00000000

Thus the correct answer is (ii) 255.0.0.0.

Note: 127.0.0.0/8 is reserved for loopback, and 255.255.255.0 is a /24 mask, not the default Class A mask.

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