Which of the following is/are restriction(s) in classless addressing ?
2015
Which of the following is/are restriction(s) in classless addressing ?
- A.
The number of addresses needs to be a power of 2
- B.
The mask needs to be included in the address to define the block
- C.
The starting address must be divisible by the number of addresses in the block
- D.
All of the above
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Correct answer: D
Answer: All of the listed restrictions apply in classless addressing (CIDR).
Block size must be a power of two. The number of addresses in a block equals 2^(32 − prefix length) for IPv4, so valid block sizes are 1, 2, 4, 8, 16, etc.
The mask (prefix length) must be included to define the block. Notation like 192.168.1.0/24 specifies how many leading bits are fixed for the network and therefore defines the block size and boundaries.
The starting (network) address must be aligned to the block size. The integer value of the starting address must be divisible by the block size. For example, a /26 block has 64 addresses; valid starting addresses include 192.168.1.0, 192.168.1.64, 192.168.1.128, and 192.168.1.192.
Because each of these conditions is required when defining CIDR blocks, the combined statement that all listed restrictions apply is correct.