VLSM Insights: Avoid IP Waste with Precision Subnetting 2025 Guide

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VLSM Insights: Avoid IP Waste with Precision Subnetting 2025 Guide

VLSM (Variable Length Subnet Masking) helps improve the use of IP address space, with applications in IPv6 internal segmentation as of June 2025.

Table of Contents

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.

Diagram Illustrating VLSM

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 NumberSubnet IDNumber of Usable IPsRemarks
Net-0115.255.240.0/25126Assign to Management
Net-1115.255.240.128/25126Assign to Sales
Net-2115.255.241.0/25126Assign to Finance
Net-3115.255.241.128/25126Required further subnetting into /26 Prefix
Net-3a115.255.241.128/2662Assign to WAN Network between the Routers
Net-3b115.255.241.192/2662Required further subnetting into /27 Prefix
Net-3b1115.255.241.192/2730Assign to Quality Control
Net-3b2115.255.241.224/2730Required further subnetting into /30 Prefix
Net-3b2A115.255.241.224/302Spare for future Expansion
Net-3b2B115.255.241.228/302Spare for future Expansion
Net-3b2C115.255.241.232/302Spare for future Expansion
Net-3b2D115.255.241.236/302Spare for future Expansion
Net-3b2E115.255.241.240/302Spare for future Expensionn
Net-3b2F115.255.241.244/302Spare for future Expansion
Net-3b2G115.255.241.248/302Spare for future Expansion
Net-3b2H115.255.241.252/302Spare for future Expansion
Net-4115.255.242.0/25128Spare for future Expansion
Net-5115.255.242.128/25128Spare for future Expansion
Net-6115.255.243.0/25128Spare for future Expansion
Net-7115.255.243.128/25Spare for future ExpansionSpare 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

Asad Ijaz

NetworkUstad's lead networking architect with CCIE certification. Specializes in CCNA exam preparation and enterprise network design. Authored 2,800+ technical guides on Cisco systems, BGP routing, and network security protocols since 2018. Picture this: I'm not just someone who writes about tech; I'm a certified expert in the field. I proudly hold the titles of Cisco Certified Network Professional (CCNP) and Cisco Certified Network Associate (CCNA). So, when I talk about networking, I'm not just whistling in the dark; I know my stuff! My website is like a treasure trove of knowledge. You'll find a plethora of articles and tutorials covering a wide range of topics related to networking and cybersecurity. It's not just a website; it's a learning hub for anyone who's eager to dive into the world of bits, bytes, and secure connections. And here's a fun fact: I'm not a lone wolf in this journey. I'm a proud member and Editor of Team NetworkUstad. Together, we're on a mission to empower people with the knowledge they need to navigate the digital landscape safely and effectively. So, if you're ready to embark on a tech-savvy adventure, stick around with me, Asad Ijaz Khattak. We're going to unravel the mysteries of technology, one article at a time!"

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