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subnet guide

CIDR Subnetting Quick Guide

CIDR notation pairs an IP address with a prefix length, such as 192.168.1.10/24. The prefix decides which bits are network bits and which bits are host bits.

What The Prefix Means

A larger prefix means a smaller subnet. For example, /24 has 256 total IPv4 addresses, while /30 has 4 total IPv4 addresses.

Important Results

  • Network address identifies the subnet.
  • Broadcast address is the last IPv4 address in the subnet.
  • Usable host range excludes network and broadcast in most traditional IPv4 LANs.
  • Wildcard mask is the inverse of the subnet mask and is common in ACLs.

Where Engineers Use It

  • Firewall rules.
  • Routing statements.
  • DHCP scopes.
  • VPN selectors.
  • Access control lists and object groups.