VLSM, or Variable Length Subnet Masking, is a computer networking technique that allows you to divide an IP network into subnets of different sizes. This means each subnet can have a different number of usable host addresses, allocated based on individual network needs.
Why Use VLSM?
Before VLSM, networks used fixed-length subnet masks (FLSM). This often led to IP address waste. For example, if you needed one subnet with 200 hosts and another with only 10, using FLSM would require both subnets to be sized for 256 hosts, even though much of the first subnet's address space would go unused, and most of the second subnet would be free, a considerable waste of IP addresses.
VLSM solves this problem by allowing you to efficiently allocate IP addresses. Key benefits include:
- Efficient IP Address Usage: Assign addresses only where needed, preventing waste.
- Hierarchical Network Design: Create a more organized and manageable network.
- Route Summarization: Simplify routing tables by grouping subnets into summary routes.
How VLSM Works
VLSM builds upon the concept of subnetting but adds the flexibility of variable-length masks. To implement VLSM, you generally follow these steps:
- Determine Network Requirements: Analyze each subnet's host requirements.
- Sort Subnets by Size: Order subnets from largest to smallest required host count.
- Allocate Address Space: Start with the largest subnet and assign the smallest subnet mask that can accommodate its hosts. Then, move to the next largest, and so on.
Example of VLSM
Let's say you have a Class C network 192.168.1.0/24 and need to divide it into the following subnets:
- Subnet A: 60 hosts
- Subnet B: 25 hosts
- Subnet C: 10 hosts
Using VLSM, you could subnet as follows:
- Subnet A (60 hosts):
192.168.1.0/26(64 hosts, subnet mask 255.255.255.192) - Addresses192.168.1.1-192.168.1.62 - Subnet B (25 hosts):
192.168.1.64/27(32 hosts, subnet mask 255.255.255.224) - Addresses192.168.1.65-192.168.1.94 - Subnet C (10 hosts):
192.168.1.96/28(16 hosts, subnet mask 255.255.255.240) - Addresses192.168.1.97-192.168.1.110
This provides adequate address space for each subnet while minimizing wasted addresses compared to using a single fixed-length subnet mask across the entire network.
VLSM and CIDR
VLSM is closely related to Classless Inter-Domain Routing (CIDR). CIDR eliminated the old classful network addressing system (Class A, B, and C) and allowed for the use of arbitrary subnet masks. VLSM builds on this flexibility by allowing different subnet masks within a single network, further improving address allocation efficiency.
Conclusion
VLSM is a valuable tool for network administrators, enabling efficient allocation and utilization of IP addresses, resulting in better network design and manageability. It is fundamental for modern network architectures where address conservation and hierarchical structure are crucial.
