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Difficulty: HardIPv4 Addressing and Subnetting

A network administrator needs to segment the IPv4 network block 10.200.16.0/2010.200.16.0/20 to accommodate multiple remote office LANs, with the largest subnet requiring support for up to 500 usable host IP addresses. Which of the following statements regarding the subnet design and IP address allocation are correct? (Select TWO.)

  1. A /23 prefix (subnet mask 255.255.254.0) is the smallest subnet mask that accommodates 500 usable host IP addresses.Answer
  2. The first usable host IP address in the second /23 subnet carved from this block is 10.200.18.1.Answer
  3. C
    A /24 prefix (subnet mask 255.255.255.0) can be assigned to accommodate 500 usable hosts by using variable-length subnet masking.
  4. D
    The broadcast address for the first /23 subnet allocated from this block is 10.200.16.255.

Answer

The correct statements are that a /23 prefix (255.255.254.0) is the smallest prefix capable of supporting 500 usable hosts, and the first usable host IP address in the second /23 subnet is 10.200.18.1.
To support 500 hosts, 9 host bits are necessary (29−2=5102^9 - 2 = 510), resulting in a /23 mask (255.255.254.0255.255.254.0). The first /23 subnet ranges from 10.200.16.010.200.16.0 to 10.200.17.25510.200.17.255 with broadcast address 10.200.17.25510.200.17.255. The second /23 subnet starts at 10.200.18.010.200.18.0, making 10.200.18.110.200.18.1 its first usable host address.

Step-by-Step Solution

1
Determine the required host bits and prefix length for 500 hosts
9 host bits are needed because 28−2=2542^8 - 2 = 254 (insufficient) and 29−2=5102^9 - 2 = 510 usable hosts (sufficient). The prefix length is 32−9=/2332 - 9 = /23 (subnet mask 255.255.254.0255.255.254.0).
Calculating usable hosts requires subtracting 2 (network and broadcast addresses) from 2h2^h.
2
Calculate subnet boundaries for consecutive /23 subnets starting from 10.200.16.0/20
Subnet 1: 10.200.16.0/2310.200.16.0/23 (Network ID: 10.200.16.010.200.16.0, First Usable: 10.200.16.110.200.16.1, Last Usable: 10.200.17.25410.200.17.254, Broadcast: 10.200.17.25510.200.17.255). Subnet 2: 10.200.18.0/2310.200.18.0/23 (Network ID: 10.200.18.010.200.18.0, First Usable: 10.200.18.110.200.18.1, Last Usable: 10.200.19.25410.200.19.254, Broadcast: 10.200.19.25510.200.19.255).
A /23 subnet increments by 2 in the third octet.
3
Evaluate the choices against the calculated results
The statement specifying /23 as the minimum mask for 500 hosts is correct. The statement identifying 10.200.18.1 as the first usable IP of the second subnet is correct. Statements proposing /24 or 10.200.16.255 as the first subnet broadcast address are incorrect.
Comparing calculated addressing parameters against each statement verifies validity.

Key Concept

IPv4 Subnetting and Host Capacity Calculation
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Topics

IPv4 Addressing and Subnetting

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