John's Quick Subnetting Worksheet
(No Binary Required!)
Practice Subnetting IPv4 https://subnetipv4.com/

  IP Address: OCTET . OCTET  . OCTET  . OCTET  
📌 Step 1: Write Down the IP Address
🖊 IP Address: ____ . ____ . ____ . ____ / ______ (CIDR Notation)

📌 Step 2: Identify the Default Subnet Mask
✔ Class A: 1 – 127 → Subnet Mask: 255.0.0.0
✔ Class B: 128 – 191 → Subnet Mask: 255.255.0.0
✔ Class C: 192 – 223 → Subnet Mask: 255.255.255.0
👉 If no bits are borrowed “GO TO STEP 8” the default subnet mask remains unchanged, and the network address is determined by the default class of the IP address:
    • Class A (1-127): Network Address = X.0.0.0 → Subnet Mask: 255.0.0.0
        ◦ Example: If the IP address is 10.45.78.120, the adjusted network address would be 10.0.0.0.
    • Class B (128-191): Network Address = X.Y.0.0 → Subnet Mask: 255.255.0.0
        ◦ Example: If the IP address is 172.16.45.200, the adjusted network address would be 172.16.0.0.
    • Class C (192-223): Network Address = X.Y.Z.0 → Subnet Mask: 255.255.255.0
        ◦ Example: If the IP address is 192.168.1.56, the adjusted network address would be 192.168.1.0.
Since no additional subnetting occurs, the network remains a single large network, maximizing the number of available host addresses.


📌 Step 3: Determine if there is an "Interesting Octet" 
🛠 The "Interesting Octet" is the Octet where subnetting (borrowing bits) occurs.
✔ Write 255 in all Octets in the subset mask before this point.

📌 Step 4: Determine Borrowed Bits (If Any) Borrowed Bits ______?

✔ If subnetting is needed, borrow bits from the “Host Portion”
✔ Use the table below to calculate the Subnet Mask Value:
Borrowed Bits
1    2   3   4  5  6  7  8
Subnet Value
128 64  32  16  8  4  2  1
Example: Borrowed bits in a /28 the first 3 octets add up to 24 so 28 would be “4” borrowed bits 24 + 4 = 28, which would add up to “240” 
✔ Add up the values below the “Borrowed Bits” in the “Table Shown    Above” to determine the "Interesting Octet."
128 + 64 + 32 + 16 = 240. So now “240” is the “Interesting Octet”  
✔ All “empty Octets” to the right, (if Any), should be replaced with ZERO
📌 IP Address:     ____ . ____ . ____ . ____
📌 Subnet Mask: ____ . ____ . ____ . ____

📌 Interesting Octet in the Subnet Mask: ______ (Circle “NONE” if no bits are borrowed)

📌 Step 5: Identify the Corresponding IP Address Octet
✔ Find the Octet in the IP Address that aligns with the Interesting Octet from Step 3.
📌 Corresponding IP Address Octet: ______ (Circle “NONE” if no bits are borrowed)

📌 Step 6: Calculate the Magic Number
🔢 Formula:
 Magic Number = 256 − ______ (The Subnet Mask’s "Interesting Octet" first non 255   	Octet)
✔ If no bits are borrowed, circle "NONE" as the magic number does not apply.
✔ If subnetting is applied, use the formula above.
📌 Magic Number: ______
Example Calculation:
✔ Subnet Mask: 255.255.240.0
✔ Interesting Octet: 240
✔ Magic Number: 256 - 240 = 16

📌 Step 7: Find the Closest Multiple
📌 Corresponding IP Address Octet: ______
📌 Magic Number: ______
👉 To find the closest multiple of the magic number that does not exceed the Corresponding Octet:
1️⃣ Divide the Corresponding IP Address Octet by the Magic Number
✔ Example: If the Coresponding IP Octet = 149 and the Magic Number = 2
149 ÷ 2 = 74.5
2️⃣ Ignore the decimal (Take only the integer part)
✔ 74
3️⃣ Multiply the result by the Magic Number
✔ 74 × 2 = 148
ATTENTION! Key Rules to Follow: 📌 Calculated Closest Multiple: ______
✔ The closest multiple must be “LESS THAN OR EQUAL TO” the corresponding IP octet.
✔ If bits are borrowed, always choose the closest valid multiple that does “NOT Exceed” 	   the “Corresponding IP Address Octet”
✔ If the closest multiple equals the corresponding IP octet, keep it as is.

✔ If the calculated multiple exceeds the corresponding IP octet, use the previous multiple.

✔ If no valid multiple exists, replace the corresponding IP octet with ZERO.
📌 Closest Multiple: ______ (Circle "NONE" if no bits are borrowed)

📌 Step 8: Adjust the Original Subnet Number (Network Address)
	                                 Original Network Address: ____ . ____ . ____ . ____ 
NO "BORROWED BITS" in the Subnet Mask, use the DEFAULT SUBNET MASK for that IP ADDRESS CLASS. Then, replace all the numbers, (OCTETS), in the IP Address that correspond to the "0s" in the Subnet Mask, with ZEROS.

IF THERE ARE BORROWED BITS “Closest Multiple”
THIS ADJUSTED NETWORK ADDRESS BELOW IS NOW YOUR NEW CALCULATED NETWORK ADDRESS
Adjusted Network Address with the Closest Multiple: ____ . ____ . ____ . ____

📌 Step 9: Calculate the Broadcast Address
📌 Adjusted Network Address with the closest multiple: ____ . ____ . ____ . ____
📌 Magic Number: ______
📌 Replaced Adjusted Corresponding IP Octet: ______
👉 Formula:
1️⃣ Carry down all the Adjusted Network Address Octets to the corresponding positions in the Broadcast Address, up to (but not including) the Replaced Adjusted Corresponding IP Octet.
2️⃣ Calculate the Broadcast Octet:
Broadcast Octet = (Adjusted Corresponding IP Address Octet) + (Magic Number − 1)
3️⃣ Bring down the calculated Broadcast Octet and insert it into its correct position.
4️⃣ Replace all Octets to the right of this Broadcast Octet with 255 to complete the Broadcast Address.
📌 Broadcast Address: ____ . ____ . ____ . ____

📌 Step 10: Determine the Host Range
📌 Adjusted Network Address with the closest multiple:   ____ . ____ . ____ . ____
✔ First Host: Network Address + 1 →                                ____ . ____ . ____ . ____
✔ Last Host: Broadcast Address Last Octet - 1 →            ____ . ____ . ____ . ____

📌 Step 11: Identify the Next Subnet
📌 Broadcast Address: ____ . ____ . ____ . ____
📌 Next Subnet = Broadcast Address + 1
✔ Rules:
🔹 If the last Octet reaches 254 or 255, increment the previous Octet and reset the last Octet
      to ZERO
🔹 If all three Octets reach 254 or 255, reset them ALL to ZERO
📌 Next Subnet: ____ . ____ . ____ . ____

📌 Step 12: Final Subnetting Results
✔ Write down all network details based on your calculations:
Network Component
Calculated Address
Network Address
____ . ____ . ____ . ____
First Host
____ . ____ . ____ . ____
Last Host
____ . ____ . ____ . ____
Broadcast Address
____ . ____ . ____ . ____
Next Subnet
____ . ____ . ____ . ____