JN0-102 · Question #298
You have been asked to use a 23-bit network mask to segment the network block 209.18.12.0. How many usable host addresses will there be per subnet?
The correct answer is A. 510. A 23-bit network mask, or /23, provides 9 host bits, allowing for 512 total IP addresses per subnet, and thus 510 usable host addresses.
Question
You have been asked to use a 23-bit network mask to segment the network block 209.18.12.0. How many usable host addresses will there be per subnet?
Options
- A510
- B1022
- C2046
- D8190
How the community answered
(50 responses)- A78% (39)
- B6% (3)
- C4% (2)
- D12% (6)
Why each option
A 23-bit network mask, or /23, provides 9 host bits, allowing for 512 total IP addresses per subnet, and thus 510 usable host addresses.
A 23-bit network mask leaves 32 - 23 = 9 host bits. The total number of IP addresses in such a subnet is 2^9 = 512. After reserving addresses for the network ID and broadcast ID, there are 512 - 2 = 510 usable host addresses.
1022 usable hosts would correspond to a /22 network, which has 10 host bits.
2046 usable hosts would correspond to a /21 network, which has 11 host bits.
8190 usable hosts would correspond to a /19 network, which has 13 host bits.
Concept tested: Calculating usable hosts in a CIDR block
Source: https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.html
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