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How to Subnet a Network

This Tutorial addresses:
This is a basic tutorial about classful subnetting.

What you need to know:



Network ID (NID) - The host bits always have values of 0.
Broadcast Address (BC) - The host bits always have values of 1.
Host Range (HR) - All host addresses in between the NID and BC. (Add 1 to the NID and subtract 1 from the BC to find the host range)

Lastly, you will need to know how to convert numbers from decimal to binary and binary to decimal.

Steps:
1. Write out the question. What class is the network address? What is the bit usage pattern of that class? Circle the octet that you will start borrowing from.

2. How many bits do you need to borrow? Draw your network line.

3. What subnetwork address are you looking for? Subtract 1 from that number and make it binary.

4. Plug that binary number (from #3) into your bits that you borrowed (from #2).

5. Now you have the NID, all you have to do is simply translate it from binary back into your decimal/octet format. That is is your NID.

6. Now find your BC by placing 1's in all the remaining host bits. Now you have the BC, all you have to do is simply translate it from binary back into your decimal/octet format. That is is your BC address.

7. Lastly, find your host range. Simply add 1 to your NID. That is your first number for the range. Simply subtract 1 from your BC address and that is your last number for the range.



Example:
Here is a sample question: "Given the network address 150.0.0.0, subnet it for up to 300 usable subnets. What is the 100th usable subnetwork address? What is the broadcast address for that subnet? What is the range of host addresses for that subnet?"

Step 1: Write out the question. 150.0.0.0 for 300 usable subnets, find the 100th. What class is it? It is a class B address since the first octet is within the 128-191 range. What is the bit usage pattern of that class? N.N.H.H. since it is a class B. (see chart above). Circle the octet that you will start borrowing from. Circle the third octet since that is the first group of host bits, that is where you'll begin.

Step 2: How many bits do you need to borrow? 9 bits since you must subnet it for 300 usable subnets. 2^8 would give you only 256 but 2^9 gives you 512. Since you must have at least 300 usable subnets, as per the question, 2^9 is the number you must use, 9 bits. Don't worry about having more possible subnets than needed, that is OK, but you may not have less since then you would not be fulfilling the requirements. Draw your network line after the 9th bit:

150.0._ _ _ _ _ _ _ _ . _|_ _ _ _ _ _ _

Step 3: What subnetwork address are you looking for? The 100th. Subtract 1 from that and make it binary. (We subtract 1 from this number because 0 is the first number in binary, 1 is the 2nd, 2 is the 3rd, etc. etc. so the 100th network would be #99). 99 in binary is 1100011.

Step 4: Plug that binary number (from #3; 1100011) into your bits borrowed (from #2; 150.0._ _ _ _ _ _ _ _ . _|_ _ _ _ _ _ _). Make sure that you start at the network line and move left, do not start at the first bit in the 3rd octet because then you will be changing the numbers value by changing it's place. If you have extra zeros to the left after you have input your binary number, simply make them a zero. Ignore the period at this point, simply start from the network line and move left.

150.0.00110001.1|_ _ _ _ _ _ _

Step 5: Now you have the NID, all you have to do is simply translate it from binary back into your decimal/octet format. That is is your NID.

150.0.00110001.1|_ _ _ _ _ _ _ is 150.0.49.128.

Step 6: Now find your BC by placing 1's in all the remaining host bits; 150.0.00110001.1|1111111. Now you have the BC, all you have to do is simply translate it from binary back into your decimal/octet format. That is is your BC address.

150.0.00110001.1|1111111 is 150.0.49.255.

Step 7: Lastly, find your host range. Add 1 to your NID. That is your first number for the range, then subtract 1 from your BC address and that is your last number for the range.

NID: 150.0.49.128+1 = 150.0.49.129
BC: 150.0.49.255-1 = 150.0.49.254
HR: 150.0.49.129-150.0.49.254.

Congratulations, you are done. Due to the length of this tutorial, I may make edits/clarifications at a later date and/or include another tutorial about number conversions (binary/decimal/hexadecimal). This is simply a first draft. Thank you for reading.

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