Subnet magic number method

Step-by-Step Algorithm Using the “Magic Number” Method

1. Identify the subnet mask

   • Convert the CIDR prefix (e.g., /21) to a dotted decimal subnet mask.

2. Find the “Magic Number”

   • Subtract the interesting octet from 256:
     • Magic Number = 256 − Subnet Mask in the interesting octet
   • The “interesting octet” is the first non-255 octet in the subnet mask.

3. Determine the Network Address

   • Find the largest multiple of the magic number less than or equal to the given IP’s value in the interesting octet.
   • Keep other octets unchanged.

4. Determine the Broadcast Address

   • Add (Magic Number - 1) to the network address in the interesting octet.
   • Change all host bits (remaining octets) to 255.

5. Find the Valid IP Range

   • First usable IP = Network Address +1
   • Last usable IP = Broadcast Address -1

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Example: Find the subnet details for 172.23.16.0/21

Step 1: Identify the subnet mask

• CIDR: /21
• Subnet mask: 255.255.248.0
  • The interesting octet is 248 (third octet).

Step 2: Find the Magic Number

• Magic Number = 256 - 248 = 8
• This means subnet blocks are in multiples of 8 in the third octet.

Step 3: Find the Network Address

• Look at the third octet in 172.23.16.0.
• The largest multiple of 8 that is ≤ 16 is 16.
• So, the network address is:
  172.23.16.0

Step 4: Find the Broadcast Address

• Add (Magic Number - 1) = 8 - 1 = 7 to the third octet of the network address: 16+7=23
• The broadcast address is:
  172.23.23.255 (last octet is 255 because all host bits are set to 1)

Step 5: Find the Valid IP Range

• First Usable IP: 172.23.16.1 (Network Address +1)
• Last Usable IP: 172.23.23.254 (Broadcast Address -1)

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Final Answer

Parameter	Value
Network Address	172.23.16.0
Broadcast Address	172.23.23.255
First Usable IP	172.23.16.1
Last Usable IP	172.23.23.254

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Summary of the Magic Number Process

1. Magic Number = 256 - subnet mask in the interesting octet
2. Network Address:
   • Largest multiple of the Magic Number ≤ given IP in the interesting octet.
3. Broadcast Address:
   • Network address + (Magic Number - 1), with all host bits set to 1.
4. Valid Range:
   • First Usable IP = Network Address +1
   • Last Usable IP = Broadcast Address -1