DEMO: VLSM Table Visualization

About this video

VLSM is a subnetting technique that allows a single IP address block to be subdivided into multiple networks of different sizes, each allocated just enough address space to meet its host requirements. Rather than applying a uniform subnet mask across all networks, VLSM lets network engineers tailor each subnet individually, making it a critical skill for efficient IPv4 address management in real-world network design. This content walks through a complete VLSM allocation using the 192.168.20.0 network, broken into five subnets with host requirements of 25, 12, 8, 2, and 2. A subnetting table is used throughout as a visual reference, mapping bit borrowing to block sizes and usable host counts. The allocation process begins with the largest requirement and works downward, a deliberate ordering that prevents address overlap and ensures each subsequent subnet fits cleanly into the remaining space. For the 25-host requirement, a /27 block is selected, providing 32 addresses with 30 usable. The 12-host network is assigned a /28, yielding 14 usable addresses. The 8-host subnet also requires a /28 because a /29 would only provide 6 usable addresses, falling short of the requirement. Both 2-host networks are assigned /30 blocks, the smallest practical subnet size, each providing exactly 2 usable addresses. The result is a fully allocated, non-overlapping address plan that meets all five network requirements with minimal waste.

What you'll learn

Apply Variable Length Subnet Masking (VLSM) to efficiently allocate IP address space across multiple subnets with differing host requirements
Interpret a subnetting table to determine the appropriate subnet mask for each network based on required host counts

What's covered

VLSM Subnetting

Scenario Requirements
- Base Network 192.168.20.0
- Five Networks Needed
- Host Counts Per Network
  - 25, 12, 8, 2, 2 Hosts
Subnetting Table Method
- Start Largest Network First
- Avoid Address Overlap
- Minimize Wasted Addresses
Subnet Allocations
- 25 Hosts Slash 27
  - 32 Address Block
- 12 Hosts Slash 28
  - 16 Address Block
- 8 Hosts Slash 28
  - 14 Usable Addresses
- Two Hosts Slash 30
  - 4 Addresses 2 Usable
Key VLSM Concepts
- Borrow Bits for Subnets
- Variable Length Subnet Mask
- Efficient Address Use

Aligned to

Cisco CCNA

1.6 Configure and verify IPv4 addressing and subnetting

CompTIA Network+

1.7 Given a scenario, use appropriate IPv4 network addressing

Cisco CCST Networking

2.2 Identify IPv4 addresses and subnet formats

CompTIA A+ Core 1

2.6 Compare and contrast common network configuration concepts

Key terms

Variable Length Subnet Mask: A subnetting technique that allows a network administrator to divide an address space into subnets of different sizes by using different prefix lengths, allocating address space more efficiently than fixed-length subnetting. VLSM requires a classless routing protocol such as OSPF, EIGRP, or RIPv2 that carries prefix length information in routing updates.
Subnet: A logical subdivision of an IP network, used to organize and segment network traffic.
Subnet Mask: A 32-bit number that divides an IP address into network and host portions.
IP Address: A numerical label assigned to each device connected to a network that uses the Internet Protocol.
Classless Inter-Domain Routing: A method for allocating IP addresses and expressing routing prefixes that replaces the older classful addressing scheme by using variable-length prefix notation (e.g., /24). CIDR enables more efficient use of IPv4 address space and reduces the size of internet routing tables through route aggregation.

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