VLSM (Variable Length Subnet Masking) helps improve the use of IP address space, with applications in IPv6 internal segmentation as of June 2025.
Using a variable-length subnet mask, we can assign LAN and WAN segments without wasting IP addresses. As shown in the scenario in the Figure below, the hosts in each subnetwork will be assigned a subnet according to the number of hosts to decrease waste.
First, we can assign the first three subnets of the previous lesson to the management, finance, and sales departments, using ip subnet-zero to include 115.255.240.0/25 as a usable subnet.
We can subnet the Net-3 into /26, /27, and 8x /30 networks. The network with the/26 prefix can accommodate up to 62 hosts, and the network with the/27 prefix can accommodate up to 30 hosts. In comparison, the network with the/30 prefix is generally used for point-to-point connectivity. The figure below illustrates the assigned IP addresses to the scenario we discussed in the previous lesson. Still, you can see that we have saved many IP addresses for future use.
VLSM Chart
An addressing chart is very important for identifying the blocks of addresses already used and available for future use, as shown in the table below. The VLSM chart helps the network administrator avoid assigning addresses that have previously been allocated.
| Sub Net Number | Subnet ID | Number of Usable IPs | Remarks |
|---|---|---|---|
| Net-0 | 115.255.240.0/25 | 126 | Assign to Management |
| Net-1 | 115.255.240.128/25 | 126 | Assign to Sales |
| Net-2 | 115.255.241.0/25 | 126 | Assign to Finance |
| Net-3 | 115.255.241.128/25 | 126 | Required further subnetting into /26 Prefix |
| Net-3a | 115.255.241.128/26 | 62 | Assign to WAN Network between the Routers |
| Net-3b | 115.255.241.192/26 | 62 | Required further subnetting into /27 Prefix |
| Net-3b1 | 115.255.241.192/27 | 30 | Assign to Quality Control |
| Net-3b2 | 115.255.241.224/27 | 30 | Required further subnetting into /30 Prefix |
| Net-3b2A | 115.255.241.224/30 | 2 | Spare for future Expansion |
| Net-3b2B | 115.255.241.228/30 | 2 | Spare for future Expansion |
| Net-3b2C | 115.255.241.232/30 | 2 | Spare for future Expansion |
| Net-3b2D | 115.255.241.236/30 | 2 | Spare for future Expansion |
| Net-3b2E | 115.255.241.240/30 | 2 | Spare for future Expensionn |
| Net-3b2F | 115.255.241.244/30 | 2 | Spare for future Expansion |
| Net-3b2G | 115.255.241.248/30 | 2 | Spare for future Expansion |
| Net-3b2H | 115.255.241.252/30 | 2 | Spare for future Expansion |
| Net-4 | 115.255.242.0/25 | 128 | Spare for future Expansion |
| Net-5 | 115.255.242.128/25 | 128 | Spare for future Expansion |
| Net-6 | 115.255.243.0/25 | 128 | Spare for future Expansion |
| Net-7 | 115.255.243.128/25 | Spare for future Expansion | Spare for future Expensionn |
VLSM’s Role in Modern Network Design
variable-length subnet mask optimizes IP usage in today’s dynamic networks, supporting growth and IPv6 transitions as of June 2025, making it indispensable for CCNA/CCNP professionals.
Troubleshooting VLSM
- Error 1: Overlapping subnets (e.g., /26 and /27). Adjust ranges with the show ip route.
- Error 2: Insufficient hosts (e.g., /30 for LAN). Use /27. Verify with show running-config.
Multi-Site VLSM Example
For 115.255.240.0/22 across 3 sites:
- Site 1 (/25, 126 hosts): 115.255.240.0 – 115.255.240.127.
- Site 2 (/27, 30 hosts): 115.255.240.128 – 115.255.240.159.
- Site 3 (/30, 2 hosts): 115.255.240.160 – 115.255.240.163.
FAQs
What is VLSM and how does it improve IP address usage?
VLSM (Variable Length Subnet Masking) optimizes IP space by assigning subnets (e.g., /25, /30) based on host needs, reducing waste, a CCNA essential.
How do I determine the subnet mask for 126 hosts using VLSM?
Use 2^h-2 ≥ 126, requiring 7 host bits (2^7-2 = 126), resulting in a /25 mask, a key skill as of June 30, 2025.
What is the role of /30 subnets in VLSM?
/30 subnets (2 usable hosts) are used for point-to-point WAN links, minimizing IP waste in router connections.
How does VLSM support scalability in large networks?
VLSM allows variable subnet sizes (e.g., /25 for 126 hosts, /30 for 2 hosts), supporting growth, critical for CCNP designs.
What Cisco commands verify VLSM configurations?
Use show ip interface brief to check IPs and show running-config to confirm subnet masks.
How does VLSM apply to IPv6 networks?
IPv6 uses fixed /64 subnets, but VLSM concepts adapt for internal segmentation within larger prefixes as of June 2025.
What are common mistakes when implementing VLSM?
Errors include overlapping subnets or insufficient hosts; troubleshoot with show ip route to resolve.
