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Difficulty: MediumIPv4 Addressing and Subnetting

A network engineer is configuring interface IP addresses for a cluster of management servers assigned to the 10.100.80.0/2310.100.80.0/23 IPv4 subnet. Which two IPv4 addresses are valid, assignable host addresses within this subnet? (Select two.)

  1. 10.100.80.25510.100.80.255Answer
  2. 10.100.81.25410.100.81.254Answer
  3. C
    10.100.81.25510.100.81.255
  4. D
    10.100.82.110.100.82.1

Answer

The addresses 10.100.80.25510.100.80.255 and 10.100.81.25410.100.81.254 are valid, assignable host addresses in the 10.100.80.0/2310.100.80.0/23 subnet.
For the network block 10.100.80.0/2310.100.80.0/23, the 9 host bits span from 10.100.80.010.100.80.0 to 10.100.81.25510.100.81.255. The network address is 10.100.80.010.100.80.0 (all host bits 0), and the broadcast address is 10.100.81.25510.100.81.255 (all host bits 1). The valid usable host IP range is 10.100.80.110.100.80.1 to 10.100.81.25410.100.81.254. Therefore, both 10.100.80.25510.100.80.255 and 10.100.81.25410.100.81.254 fall within the valid host range.

Step-by-Step Solution

1
Determine the subnet mask and block size for a /23 prefix
A /23 prefix uses a subnet mask of 255.255.254.0255.255.254.0, providing 9 host bits (29=5122^9 = 512 total IP addresses) and a block size of 2 in the third octet.
Calculating block size allows identification of the subnet boundaries.
2
Calculate the network and broadcast addresses for 10.100.80.0/2310.100.80.0/23
Network address is 10.100.80.010.100.80.0. The broadcast address is the last address before the next subnet (10.100.82.010.100.82.0), which is 10.100.81.25510.100.81.255.
Host addresses cannot be assigned the network ID or broadcast ID.
3
Identify the usable host address range
The assignable host range extends from 10.100.80.110.100.80.1 through 10.100.81.25410.100.81.254.
Usable host range includes all IPs between the network ID and broadcast ID.
4
Evaluate the given options against the usable host range
10.100.80.25510.100.80.255 and 10.100.81.25410.100.81.254 both fall strictly within the assignable range [10.100.80.1,10.100.81.254][10.100.80.1, 10.100.81.254].
Addresses within this inclusive range are valid host assignments.

Key Concept

IPv4 Subnet Boundaries and Host Address Ranges for Non-Byte-Aligned Prefixes
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Topics

IPv4 Addressing and Subnetting

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