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01-03: Exercises

Question

An IPv4 host has the following configuration:

  • IP address: 192.168.89.130
  • Subnet mask: 255.255.255.224 (/27)

Based on this information, answer the following:

  1. What is the magic number (block size)?
  2. What is the network ID of the first subnet?
  3. What are the network IDs of:
  4. the second subnet
  5. the last subnet
  6. What is the network ID of the subnet to which the given host belongs?
  7. What is the broadcast address of that subnet?
  8. What is the usable host range of that subnet?

Solution

Step 1: Magic number (Block Size)

Subnet mask = 255.255.255.224

Magic number = 256 − 224 = 32

👉 (1) Magic number = 32

Step 2: Network ID of the first subnet

This is a Class C network:

  • Default network portion = first 3 octets → 192.168.89

Thus, first subnet starts from: 192.168.89.0

👉 (2) First subnet network ID = 192.168.89.0

Step 3: Network IDs of subnets

Using block size 32, subnet IDs increase by 32:

  • 1st → 192.168.89.0
  • 2nd → 192.168.89.32
  • 3rd → 192.168.89.64
  • 4th → 192.168.89.96
  • 5th → 192.168.89.128
  • 6th → 192.168.89.160
  • 7th → 192.168.89.192
  • 8th → 192.168.89.224

👉 (3) Network IDs: - Second subnet → 192.168.89.32
- Last subnet → 192.168.89.224

Step 4: Network ID of the host’s subnet

Host IP = 192.168.89.130

Since block size = 32, subnet boundaries occur at: 0, 32, 64, 96, 128, 160, ...

The largest value ≤ 130 is 128.

Thus:

👉 (4) Network ID = 192.168.89.128

Step 5: Broadcast address of the host’s subnet

The host belongs to the subnet that starts at 192.168.89.128.

Since block size = 32,

  • Next subnet starts from = 192.168.89.128 + 32 = 192.168.89.160

So, the last address of the current subnet = 192.168.89.160 − 1 = 192.168.89.159

The last address of this subnet is the broadcast address.

👉 (5) Broadcast address = 192.168.89.159

Step 6: Host range of the subnet

  • Network ID = 192.168.89.128
  • Broadcast ID = 192.168.89.159

Usable host range: 192.168.89.129 to 192.168.89.158

👉 (6) Host range = 192.168.89.129 to 192.168.89.158

Final Summary

  1. Magic number = 32

  2. First subnet network ID = 192.168.89.0

  3. Network IDs:

  4. Second subnet → 192.168.89.32

  5. Last subnet → 192.168.89.224

  6. Host’s subnet network ID = 192.168.89.128

  7. Host’s Broadcast address = 192.168.89.159

  8. Host range = 192.168.89.129 to 192.168.89.158