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Netmask Calculator

Convert between CIDR notation, decimal subnet masks, binary formats, and wildcard masks

Netmask Conversion

Input Format

CIDR Prefix Length (e.g., 24)
Decimal Subnet Mask (e.g., 255.255.255.0)

How to Use the Netmask Calculator

Getting Started

  • Step 1: Choose input format - CIDR prefix or decimal subnet mask
  • Step 2: Enter your netmask value in the selected format
  • Step 3: Click "Convert Netmask" to see all format conversions
  • Step 4: Review binary, wildcard, and network information

Key Concepts

  • CIDR Notation: Slash notation indicating number of network bits (e.g., /24)
  • Subnet Mask: 32-bit value that separates network and host portions
  • Wildcard Mask: Inverse of subnet mask, used in access control lists
  • Network/Host Bits: Division of 32 bits between network and host identification

Best Practices

  • Documentation: Always document subnet masks used in network designs
  • Consistency: Use consistent notation throughout network configurations
  • Planning: Choose appropriate subnet sizes based on host requirements
  • Validation: Verify subnet mask calculations before implementation

Network Guidelines

  • Class A: /8 networks for very large organizations (16M+ hosts)
  • Class B: /16 networks for medium organizations (65K hosts)
  • Class C: /24 networks for small organizations (254 hosts)
  • VLSM: Variable Length Subnet Masking for efficient IP allocation

Frequently Asked Questions

What's the difference between subnet mask and wildcard mask?

A subnet mask has 1s for network bits and 0s for host bits, while a wildcard mask is the inverse - 0s for network bits and 1s for host bits. Wildcard masks are commonly used in router access control lists and OSPF area configurations.

Why do we need different netmask formats?

Different network devices and configurations use different formats. CIDR notation (/24) is concise and modern, decimal format (255.255.255.0) is traditional and human-readable, binary shows exact bit patterns, and wildcard masks are used in specific router configurations.

How do I choose the right subnet mask for my network?

Choose based on the number of hosts needed. Count required hosts, add growth margin, then select the next larger subnet size. Remember that network and broadcast addresses reduce usable hosts by 2.

What does /30 mean and when is it used?

/30 provides only 4 total addresses (2 usable), making it perfect for point-to-point links between routers where you only need 2 IP addresses. It's the most efficient subnet for router-to-router connections.

Can I use any combination of 1s and 0s in a subnet mask?

No, valid subnet masks must have consecutive 1s followed by consecutive 0s. For example, 11111111.11110000.00000000.00000000 is valid, but 11111111.10101010.00000000.00000000 is not.

How does VLSM help with IP address efficiency?

Variable Length Subnet Masking allows using different subnet sizes within the same network. You can use /30 for point-to-point links, /28 for small LANs, and /24 for larger segments, maximizing IP address utilization.

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