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Difficulty: MediumIPv4 Addressing and Subnetting

A network administrator needs to assign an IPv4 address to the router interface serving a newly established remote VLAN. The subnet is carved from the 10.200.16.0/2010.200.16.0/20 parent address space and must support a maximum of 2828 end-user host devices using the most efficient subnet mask possible. The policy requires configuring the router interface with the last usable host IP address in the first available subnet block. Which IP address and subnet mask combination must be assigned to the router interface?

  1. 10.200.16.30 255.255.255.224Answer
  2. B
    10.200.16.31 255.255.255.224
  3. C
    10.200.16.14 255.255.255.240
  4. D
    172.32.16.30 255.255.255.224

Answer

10.200.16.30 with subnet mask 255.255.255.224
To support 28 hosts, 5 host bits are required (25−2=302^5 - 2 = 30 usable addresses), yielding a /27 prefix (subnet mask 255.255.255.224). The first subnet derived from 10.200.16.0/20 is 10.200.16.0/27, which spans from 10.200.16.0 (network ID) to 10.200.16.31 (broadcast ID). The last usable host IP address in this range is 10.200.16.30.

Step-by-Step Solution

1
Determine the required number of host bits (nn) for 2828 hosts.
Using the formula 2n−2≥282^n - 2 \ge 28, n=5n = 5 host bits because 25−2=302^5 - 2 = 30 usable addresses (32−5=2732 - 5 = 27 prefix length or subnet mask 255.255.255.224255.255.255.224).
Selecting 4 host bits (/28) provides only 14 usable hosts, which falls short of the 28 required hosts.
2
Calculate the subnet boundaries for the first /27 subnet allocated from 10.200.16.0/2010.200.16.0/20.
Network Address: 10.200.16.0/2710.200.16.0/27; Broadcast Address: 10.200.16.3110.200.16.31.
A /27 subnet has a block size of 32 in the fourth octet (256−224=32256 - 224 = 32).
3
Identify the usable host range and the last usable host IP address.
Usable host range: 10.200.16.110.200.16.1 to 10.200.16.3010.200.16.30. The last usable address is 10.200.16.3010.200.16.30.
The network ID (10.200.16.010.200.16.0) and broadcast ID (10.200.16.3110.200.16.31) cannot be assigned to network interfaces.

Key Concept

Subnet Host Calculation and Usable Address Boundaries
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Topics

IPv4 Addressing and Subnetting

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