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Difficulty: MediumIPv4 Addressing and Subnetting

A network engineer is configuring static host IP addresses for infrastructure servers within the allocated CIDR block 10.240.16.0/2110.240.16.0/21. Which of the following IP addresses are valid usable host addresses within this subnet? (Select TWO.)

  1. A
    10.240.16.010.240.16.0
  2. 10.240.17.10010.240.17.100Answer
  3. 10.240.20.25510.240.20.255Answer
  4. D
    10.240.23.25510.240.23.255

Answer

The addresses 10.240.17.10010.240.17.100 and 10.240.20.25510.240.20.255 are valid usable host IP addresses within the 10.240.16.0/2110.240.16.0/21 subnet.
For the subnet 10.240.16.0/2110.240.16.0/21, the usable host range spans from 10.240.16.110.240.16.1 through 10.240.23.25410.240.23.254. Both 10.240.17.10010.240.17.100 and 10.240.20.25510.240.20.255 fall within this valid range. Note that 10.240.20.25510.240.20.255 is valid because the broadcast address for the entire /21/21 block is 10.240.23.25510.240.23.255.

Step-by-Step Solution

1
Determine the subnet mask and block size from CIDR notation.
A /21/21 mask corresponds to 255.255.248.0255.255.248.0. The block size in the third octet is 256−248=8256 - 248 = 8.
Calculating block size establishes the boundary of the subnetwork.
2
Identify the network ID and broadcast address for the 10.240.16.0/2110.240.16.0/21 subnet.
Network ID: 10.240.16.010.240.16.0. Next subnet ID: 10.240.24.010.240.24.0. Broadcast Address: 10.240.23.25510.240.23.255.
The network address is the first address in the block, and the broadcast address is the last address before the next subnet boundary.
3
Determine the range of usable host IP addresses.
Usable host range is from 10.240.16.110.240.16.1 to 10.240.23.25410.240.23.254.
The first usable IP is Network ID +1+ 1, and the last usable IP is Broadcast ID −1- 1.
4
Evaluate the candidate options against the valid host range.
10.240.17.10010.240.17.100 and 10.240.20.25510.240.20.255 fall strictly between 10.240.16.110.240.16.1 and 10.240.23.25410.240.23.254. 10.240.16.010.240.16.0 is the Network ID and 10.240.23.25510.240.23.255 is the Broadcast ID.
Only IP addresses between the network address and broadcast address can be assigned to host interfaces.

Key Concept

Subnet Host Range Boundaries & Intermediate Octet .255 Host Addresses
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Topics

IPv4 Addressing and Subnetting

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