Subnetting classful networks (Class A, B, C) involves dividing the default classful networks into smaller subnets to meet specific network requirements. Classful networks have predefined subnet masks based on their class designation (A, B, or C). Here's how subnetting is performed for each class:
1. Class A Subnetting (1.0.0.0 to 127.255.255.255):
- Default Subnet Mask: 255.0.0.0 (/8)
- Example: 10.0.0.0/8
- To subnet a Class A network, borrow bits from the host portion to create smaller subnets.
- Subnet mask options include /9, /10, /11, …, /32.
2. Class B Subnetting (128.0.0.0 to 191.255.255.255):
- Default Subnet Mask: 255.255.0.0 (/16)
- Example: 172.16.0.0/16
- Subnetting a Class B network involves borrowing bits from the host portion.
- Subnet mask options include /17, /18, /19, …, /32.
3. Class C Subnetting (192.0.0.0 to 223.255.255.255):
- Default Subnet Mask: 255.255.255.0 (/24)
- Example: 192.168.1.0/24
- Subnetting a Class C network also involves borrowing bits from the host portion.
- Subnet mask options include /25, /26, /27, …, /32.
Steps for Subnetting:
1. Determine the number of subnets needed. 2. Calculate the number of bits required for the subnet portion based on the number of subnets (using the formula \(2^{\text{number of subnet bits}}\)). 3. Determine the number of bits available for hosts by subtracting the subnet bits from the total bits in the default subnet mask. 4. Calculate the number of hosts per subnet (using the formula \(2^{\text{number of host bits}} - 2\) to account for the network and broadcast addresses). 5. Choose appropriate subnet mask lengths (/n notation) based on the number of subnet and host bits. 6. Allocate subnet addresses, ensuring no overlap between subnets. 7. Document the subnetting scheme and apply configurations to network devices.
For example, to subnet a Class C network (192.168.1.0/24) into 4 subnets, you would borrow 2 bits for the subnet portion, resulting in subnet mask lengths of /26. This would create 4 subnets with 62 usable hosts each.
Subnetting classful networks allows for efficient use of IP address space and better organization of network resources to meet specific network requirements.