Classless Subnet Calculator

Advanced subnet calculator supporting VLSM (Variable Length Subnet Masking) and CIDR notation for flexible network planning.

Understanding Classless Subnetting

Classless Inter-Domain Routing (CIDR) eliminated the rigid classful network architecture, allowing for more efficient IP address allocation through variable length subnet masking (VLSM).

Key Concepts in Classless Subnetting

Concept Description Example
VLSM Variable Length Subnet Masking allows different subnet sizes within the same network Using /26 and /28 in same network
CIDR Notation Compact representation of network prefix length 192.168.1.0/24
Supernetting Combining multiple networks into a larger one 192.168.0.0/23 combines two /24s
Network Prefix The portion of IP that identifies the network First 24 bits in 192.168.1.0/24

VLSM Subnetting Example

Subnet Required Hosts CIDR Subnet Mask Host Range
LAN 1 100 /25 255.255.255.128 192.168.1.1-126
LAN 2 50 /26 255.255.255.192 192.168.1.129-190
Point-to-Point 2 /30 255.255.255.252 192.168.1.193-194

Frequently Asked Questions

What's the difference between classful and classless subnetting?

Classful networking used fixed classes (A, B, C) with predetermined network sizes. Classless subnetting (CIDR) allows for arbitrary network sizes, enabling more flexible and efficient address allocation.

How does VLSM improve network efficiency?

VLSM allows you to create subnets of different sizes within the same network, minimizing wasted IP addresses by precisely matching subnet sizes to actual requirements.

What is the maximum number of subnets I can create?

The maximum depends on your original network size and how small your subnets are. For example, a /24 network can be divided into:

  • 2 /25 subnets (128 hosts each)
  • 4 /26 subnets (64 hosts each)
  • 8 /27 subnets (32 hosts each)
  • ... up to 64 /30 subnets (2 hosts each)

How do I calculate the number of hosts per subnet?

The formula is: Usable hosts = 2^(32 - CIDR) - 2. For example, a /26 subnet has 2^(32-26) - 2 = 64 - 2 = 62 usable hosts.

What are the private IP address ranges?

The private IP ranges (not routable on the public internet) are:

  • 10.0.0.0/8
  • 172.16.0.0/12
  • 192.168.0.0/16

How do I handle subnetting for point-to-point links?

For point-to-point connections (like router links), use /31 networks (which have 2 usable addresses) or /30 networks (with 2 usable addresses after accounting for network and broadcast).

What's the best way to document my subnet plan?

Create a subnetting table that includes:

  • Subnet purpose/name
  • Network address and CIDR
  • IP range
  • Gateway address
  • VLAN ID (if applicable)