Home CCNA VLSM Insights: Avoid IP Waste with Precision Subnetting 2025 Guide
CCNA

VLSM Insights: Avoid IP Waste with Precision Subnetting 2025 Guide

Asad Ijaz July 28, 2019 3 min read
Diagram Showing Vlsm Subnetting With Ip Ranges And Host Allocations For Departments Like Sales, Hr, And Management.

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 Insights: Avoid Ip Waste With Precision Subnetting 2025 Guide 3

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

About This Content

Author Expertise: 15 years of experience in NetworkUstad's lead networking architect with CCIE certification. Specializes in CCNA exam preparation and enterprise network…. Certified in: BSC, CCNA, CCNP
Avatar Of Asad Ijaz

Asad Ijaz

NetworkUstad Contributor

Related Articles

Isometric Network Diagram Showing A Glowing Ppp Data Pipeline Connecting A Router To An Isp Cloud, With Labeled Ncp And Ipcp Gateways And Flowing Ipv4/Ipv6 Packets On A Dark Blue Grid Background.
CCNA

The Invisible Handshake: A Deep Dive into Network Control Protocols (NCP)

Imagine you are in a room filled with people speaking different languages. For any meaningful conversation to happen, everyone must first agree on a common language. In the world of networking, this is the role of protocols. However, before a web browser can speak HTTP (the language of the web) or an email client can...

Asad Ijaz 9 min read
Illustration Of Cisco Access Control Lists Showing Traffic Filtering From Cloud To Router Denying Access To A Computer Monitor.
CCNA

Comprehensive Guide to Access Control Lists (ACLs) in Cisco Networking: Concepts, Types, Operations, Wildcard Masking, Creating, Placemnet and Differences (2025)

An Access Control List (ACL) is a critical feature in Cisco networking, used for security and traffic management. It consists of a set of rules designed to filter network traffic by specifying which users or systems are granted access to network resources and what operations they can perform. ACLs are commonly configured on devices with...

Asad Ijaz 18 min read
Ospf Dr And Bdr Election Diagram Showing Router1 As Bdr And Router2 As Dr Based On Default Priorities And Router Ids In A Multi-Access Network.
CCNA

Understanding OSPF Neighbor Adjacencies, DR/BDR Election, and Database Synchronization

If a router interface is configured with OSPF, the router must determine if another OSPF neighbor is on the link. To do this, the router forwards a Hello packet containing its router ID to all OSPF-enabled interfaces as multicast. The OSPF process uses the router ID to recognize every router individually in the OSPF area....

Asad Ijaz 6 min read
Subscribe
Subscribe