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Difficulty: MediumIPv4 Addressing and Subnetting

A network engineer is configuring IP address assignments for client workstations within a corporate VLAN allocated the 10.150.32.0/2110.150.32.0/21 IPv4 block. Which two IP addresses are valid host addresses that can be assigned to devices on this subnet?

  1. 10.150.35.25510.150.35.255Answer
  2. 10.150.39.25410.150.39.254Answer
  3. C
    10.150.39.25510.150.39.255
  4. D
    10.150.40.010.150.40.0

Answer

The valid host addresses are 10.150.35.25510.150.35.255 and 10.150.39.25410.150.39.254.
For the prefix /21/21, the subnet mask is 255.255.248.0255.255.248.0, giving a block size of 88 in the third octet. The subnet 10.150.32.0/2110.150.32.0/21 ranges from network address 10.150.32.010.150.32.0 to broadcast address 10.150.39.25510.150.39.255. The usable host range is 10.150.32.110.150.32.1 through 10.150.39.25410.150.39.254. Both 10.150.35.25510.150.35.255 and 10.150.39.25410.150.39.254 fall within this usable range.

Step-by-Step Solution

1
Determine the subnet mask and block size for a /21 prefix
Prefix /21 corresponds to subnet mask 255.255.248.0. The third octet block size is 256 - 248 = 8.
Calculating block size in the interesting octet establishes subnet boundaries.
2
Calculate the network and broadcast addresses for the 10.150.32.0/21 subnet
Network address is 10.150.32.0. The next subnet starts at 10.150.40.0. Therefore, the broadcast address is 10.150.39.255.
Subnet boundaries define the start and end of the address block.
3
Determine the usable host IP address range
Usable host range spans from 10.150.32.1 to 10.150.39.254 inclusive.
Usable host IPs exclude the network ID (all host bits 0) and broadcast ID (all host bits 1).
4
Evaluate the given options against the usable host range
10.150.35.255 and 10.150.39.254 lie between 10.150.32.1 and 10.150.39.254. Address 10.150.39.255 is the broadcast IP and 10.150.40.0 belongs to the next subnet.
Verifies which IP addresses fall within the valid host envelope.

Key Concept

Usable host address range determination for IPv4 subnets with non-byte-aligned prefixes
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Topics

IPv4 Addressing and Subnetting

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