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IPv4 Wildcard Mask

Contiguous mask only

Result: Netmask 255.255.255.0 ↔ Wildcard 0.0.0.255

About This Tool

The IPv4 Wildcard Mask converter is a specialized tool that converts between traditional subnet masks and wildcard masks used in Cisco IOS Access Control Lists (ACLs) and OSPF configurations. Wildcard masks are the inverse of subnet masks and are essential for configuring network device access control. This bidirectional tool makes it easy to translate between the two formats, eliminating manual calculation errors when configuring routers and switches.

How to Use

  1. Enter a subnet mask in dotted-decimal format (e.g., 255.255.255.0) in the Netmask field, OR
  2. Enter a wildcard mask (e.g., 0.0.0.255) in the Wildcard field
  3. The tool automatically converts and displays the corresponding value in the other field
  4. Copy the result for use in your Cisco IOS or other network device configurations
  5. Note: Only contiguous masks (standard subnet masks) are supported

Features

  • Bidirectional conversion between netmask and wildcard mask
  • Real-time conversion as you type in either field
  • Automatic synchronization between both input fields
  • Validates input for proper mask format
  • Supports all standard contiguous subnet masks
  • Ready-to-use output for Cisco ACL configurations
  • Clean interface focused on quick conversions

Common Use Cases

  • Converting subnet masks for Cisco IOS ACL configurations
  • Preparing wildcard masks for OSPF network statements
  • Configuring EIGRP network advertisements
  • Setting up access control lists on Cisco routers and switches
  • Translating network documentation between mask formats
  • Verifying wildcard mask syntax in existing configurations
  • Learning the relationship between subnet and wildcard masks
  • Troubleshooting ACL matching issues by examining mask values

Technical Details

Wildcard masks are the binary inverse (logical NOT) of subnet masks. Where a subnet mask has a "1" bit, the wildcard mask has a "0", and vice versa.

Conversion Formula: Wildcard Mask = 255.255.255.255 - Subnet Mask

Common Conversions:

  • Subnet Mask 255.255.255.255 (/32) → Wildcard 0.0.0.0 (exact host match)
  • Subnet Mask 255.255.255.252 (/30) → Wildcard 0.0.0.3 (4 addresses)
  • Subnet Mask 255.255.255.0 (/24) → Wildcard 0.0.0.255 (256 addresses)
  • Subnet Mask 255.255.0.0 (/16) → Wildcard 0.0.255.255 (65,536 addresses)
  • Subnet Mask 255.0.0.0 (/8) → Wildcard 0.255.255.255 (16,777,216 addresses)
  • Subnet Mask 0.0.0.0 (/0) → Wildcard 255.255.255.255 (any address)

Usage in Cisco ACLs:

In a Cisco ACL, "0" means "must match" and "1" means "don't care". For example:

  • access-list 10 permit 192.168.1.0 0.0.0.255 matches all hosts in 192.168.1.0/24
  • access-list 10 permit 10.0.0.0 0.255.255.255 matches all hosts in 10.0.0.0/8

This tool ensures accurate conversion for all contiguous masks, preventing configuration errors in production networks.