Subnet Mask Cheat Sheet

Subnet Mask Cheat Sheet is a web-based reference tool that provides instant lookup of CIDR prefix, subnet mask, total IPs, usable hosts, wildcard mask, and class information for all IPv4 subnets from /0 to /32.

What is the Subnet Mask Cheat Sheet?

The Subnet Mask Cheat Sheet is a single-page online calculator and reference that lists every possible IPv4 subnet mask in CIDR notation alongside its dotted-decimal form, hexadecimal representation, total IP count, usable host count, wildcard mask, host bits, and legacy class designation. It takes a CIDR prefix (e.g., /24) and outputs the corresponding subnet mask (255.255.255.0) and all derived metrics. The page also includes private IP ranges (RFC 1918), common mask shortcuts, an explanation of classful addressing, and a FAQ section. No login or software installation is required; the tool runs entirely in the browser.

Key Features

• Complete CIDR table — All 33 prefixes from /0 to /32 with columns for subnet mask, hex, total IPs (e.g., /24 = 256), usable hosts (e.g., 254), wildcard mask (e.g., 0.0.0.255), host bits, and class (A/B/C).
• Wildcard mask values — Each row includes the wildcard mask (inverse of subnet mask) used in ACLs and OSPF configurations.
• Private range references — Lists the three RFC 1918 ranges: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
• /31 exception note — Documents RFC 3021, which allows /31 subnets on point-to-point links to use both addresses as hosts (no network/broadcast waste).
• Legacy classful info — Provides the default masks for Class A (/8), B (/16), C (/24) and the full address range for each class.
• Built-in FAQ — Answers common questions such as "How many usable hosts in a /23?" (510) and the difference between /24 and 255.255.255.0.

Who should use the Subnet Mask Cheat Sheet?

Network engineers designing subnets or configuring routing tables use the tool to quickly verify host counts (e.g., a /29 yields 6 usable addresses). System administrators setting up DHCP scopes or access control lists (ACLs) reference the wildcard mask column for Cisco and Linux firewall rules. Students studying for networking certifications (CCNA, CompTIA Network+) rely on the table to memorize CIDR-to-mask mappings and practice subnetting calculations.

What can you do with the Subnet Mask Cheat Sheet?

• Design point-to-point links — Identify that a /30 provides exactly 2 usable hosts (commonly used for router-to-router links) or use the /31 exception when RFC 3021 applies.
• Calculate host capacity quickly — For a /20 subnet, the table shows 4,096 total IPs and 4,094 usable hosts without manual math.
• Plan internal private networks — Cross‑reference the RFC 1918 ranges to ensure chosen subnets (e.g., 192.168.1.0/24) don't conflict with public IP assignments.

FAQ

What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation uses an IP address followed by a slash and a number (e.g., /24) to indicate how many bits form the network prefix. The Subnet Mask Cheat Sheet lists every possible prefix, so you can instantly find the corresponding dotted-decimal mask and host range.

Why use /30 for point-to-point links?

A /30 subnet provides exactly 2 usable host addresses (4 total IPs minus network and broadcast). This minimizes wasted IP space for router-to-router connections. The cheat sheet's row for /30 shows these specifics: mask 255.255.255.252, wildcard 0.0.0.3, and class "1/2 C".

What is the difference between /24 and 255.255.255.0?

There is no technical difference — /24 is the CIDR shorthand for a subnet mask of 255.255.255.0. The cheat sheet displays both formats side by side for every prefix.

How many usable hosts are in a /23?

A /23 subnet contains 512 total IP addresses. Subtracting the network and broadcast addresses leaves 510 usable hosts. The table confirms this: 510 in the "Usable" column and a wildcard mask of 0.0.1.255.