CIDR Subnet Guide

Convert CIDR blocks to IP ranges and master subnetting for IPv4 — in one streamlined resource.

What is CIDR?

Classless Inter-Domain Routing (CIDR) is the system that replaced the old class-A/B/C scheme in 1993. A network is written as

address/prefix-length → 10.0.0.0/24

The prefix length (the number after the slash) tells routers how many of the 32 bits belong to the network portion:

  • 10.0.0.0/24 → 24 network bits + 8 host bits → 256 addresses (254 usable)
  • 10.0.0.0/30 → 30 network bits + 2 host bits → 4 addresses (2 usable)

CIDR lets ISPs advertise aggregated “supernets” such as 203.0.113.0/22 instead of listing a thousand individual /24 routes—shrinking global routing tables and conserving address space.

CIDR → IP Range Converter

Paste any block—say 192.168.15.64/27—into the calculator. Behind the scenes it:

  1. Bitwise-ANDs the IP with the mask to find the network address.
  2. Inverts the mask and ORs it with the IP to get the broadcast address.
  3. Offsets ±1 to reveal the first and last usable hosts.

Hit Copy Share Link to bookmark or send an exact result.

Subnetting Tutorial — Quick Steps

Subnetting seems daunting until you break it into four predictable moves:

  1. Convert the prefix /26 → 255 .255 .255 .192
  2. Find block size 256 − 192 = 64
  3. List subnet starts 0, 64, 128, 192
  4. Pick your block Usable hosts = block size − 2 (62 in a /26)

Example

Split 172.16.0.0/16 into at least 10 equal subnets.

Need ≥10 blocks → 4 extra bits (2⁴ = 16)

New prefix = /20. Each /20 holds 4094 hosts.

Ranges begin 172.16.0.0, 172.16.16.0, 172.16.32.0 … to 172.16.240.0.

IPv6 Subnetting Basics

The slash notation is identical in IPv6—just with 128-bit addresses.

  • /64  is the standard LAN size → 18 quintillion addresses

  • Providers often delegate:

    • /48  — 65 536 customer /64s
    • /56  — 256 /64s
    • /60  — 16 /64s

Toggle IPv6 in the calculator and try 2001:db8::/48 to explore the range graphically.

Further Reading & Tools

  • RFC 4632 — Classless Inter-Domain Routing for IPv6
  • RFC 1519 — CIDR Address Allocation for IPv4
  • cidr.xyz source code