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Learn subnetting

Tutorials, reference tables, practice quizzes and a downloadable cheat sheet — for CCNA prep and daily work.

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CIDR Quick Reference

All CIDR prefixes /0 to /32
CIDRSubnet maskWildcardTotal IPsUsable
Glossary
CIDR — Classless Inter-Domain Routing. Notation like 192.168.0.0/24.
Prefix length — Number after the slash. Indicates how many bits are network bits.
Subnet mask — Dotted-decimal representation of the prefix (e.g., 255.255.255.0 = /24).
Wildcard mask — Inverse of subnet mask. Used in Cisco ACLs.
Network address — First IP in a subnet. Reserved, not assignable.
Broadcast address — Last IP in a subnet. Reserved for broadcasts.
VLSM — Variable Length Subnet Masking. Mixing different prefix sizes within one network.
Supernet — A summary route that aggregates several smaller CIDR blocks.
RFC 1918 — Document defining private IPv4 ranges.
CGNAT — Carrier-Grade NAT, RFC 6598 range (100.64.0.0/10).
ULA — Unique Local Address. IPv6 equivalent of RFC 1918 (fc00::/7).
GUA — Globally Unique Address. Public IPv6 (2000::/3).
Bogon — IP address in a range that should never appear on the public internet.
Subnetting Cheat Sheet
Download Cheatsheet

A one-page reference for CCNA / Network+ exam prep and daily work.

Common prefixes

/24255.255.255.0256Small office
/23255.255.254.0512Medium office
/22255.255.252.01024Building
/16255.255.0.065,536Campus / VPC
/12255.240.0.01,048,576Region
/8255.0.0.016,777,216Org

Powers of 2

2^416/28
2^532/27
2^664/26
2^7128/25
2^8256/24
2^101,024/22
2^1665,536/16