Subnetting a Class B IP address involves dividing the network into smaller sub-networks (subnets) to optimize the use of IP addresses and enhance network management. Class B IP addresses use a default subnet mask of 255.255.0.0 (or /16). Here’s how to go about subnetting a Class B IP address:
1. Understand Class B Addressing:
- Class B addresses range from 128.0.0.0 to 191.255.255.255.
- The default subnet mask is 255.255.0.0 (or /16), which means the first 16 bits are for the network part, and the remaining 16 bits are for the host part.
2. Determine Subnet Requirements:
- Identify how many subnets you need. This can be based on geographic locations, departments, or any criteria that suit your organizational needs.
- Determine the maximum number of hosts required per subnet.
3. Calculate Subnet Mask:
To create subnets, you can "borrow" bits from the host portion of the IP address. Here’s how to do this:
- Identify the number of bits to borrow: Use the formula
2^n >= number of required subnets, wherenis the number of bits borrowed. - Calculate hosts per subnet: Once you know how many bits you are using for subnetting, the remaining bits will determine how many hosts you can have per subnet using the formula
2^(remaining bits) - 2(to account for the network and broadcast addresses).
4. Example Calculation:
Scenario: You need 8 subnets.
- Borrow bits:
- If you need 8 subnets, you need to borrow 3 bits (2^3 = 8).
- New subnet mask:
- The original Class B mask is /16. Borrowing 3 bits gives you a new subnet mask of /19 (16 + 3).
- In decimal: 255.255.248.0 (because 19 bits gives us 11111111.11111111.11111000.00000000).
Hosts per subnet:
- Remaining bits for hosts = 32 - 19 = 13.
- Maximum hosts:
2^13 - 2 = 8192 - 2 = 8190 hosts per subnet.
5. Calculate Subnets:
Now that you have the subnet mask, you can calculate the possible subnets:
- If your Class B IP address is 172.16.0.0/19, the subnets would be:
- 172.16.0.0/19
- 172.16.8.0/19
- 172.16.16.0/19
- 172.16.24.0/19
- 172.16.32.0/19
- 172.16.40.0/19
- 172.16.48.0/19
- 172.16.56.0/19
6. Document the Configuration:
Always document your subnetting scheme by noting the subnet addresses, subnet masks, the range of hosts available for each subnet, and any other relevant details.
7. Implement and Test:
Once documented, implement the configuration on your network devices and test connectivity between subnets to ensure proper segmentation.
Summary:
- Start with a Class B address (e.g., 172.16.0.0).
- Determine the number of subnets needed.
- Calculate necessary bits to borrow.
- Compute the new subnet mask and allowed hosts.
- Create the subnet plan and document it.
By carefully planning and following these steps, you can efficiently subnet a Class B IP address to meet your network’s needs.