The Subnet Mask Cheat Sheet is a free online reference tool that provides an instant lookup table for CIDR notation, subnet masks, total and usable IP addresses, and wildcard masks for all IPv4 prefixes from /0 to /32.
What is the Subnet Mask Cheat Sheet?
The Subnet Mask Cheat Sheet is a web-based CIDR calculator and reference guide for network professionals. It takes a CIDR prefix (e.g., /24) and displays its corresponding subnet mask in dotted-decimal and hex, total IPs, usable hosts, wildcard mask, host bits, and legacy class. No input is required from the user—the entire table is precomputed for all 33 possible IPv4 prefix lengths. The site is hosted at subnetmaskcheatsheet.com and is maintained as a free public resource.
Key Features
- Comprehensive CIDR table — Lists every prefix from /0 (default) to /32 (host), each with subnet mask (e.g., 255.255.255.0 for /24), hex value, total IPs (2^(32-prefix)), usable hosts (Total‑2), wildcard mask (inverse of subnet mask), host bits, and legacy class designation.
- Calculation logic displayed — Shows formulas for total IPs (2^(32-prefix)) and usable hosts (2^(32-prefix)-2), with a note that /31 subnets use both addresses as hosts per RFC 3021.
- Private range documentation — Lists the three RFC 1918 private IP ranges: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
- Legacy classful addressing — Includes a table for Class A, B, and C with default masks and address ranges, for compatibility with older systems.
- Common masks quick reference — Displays one‑line entries for /32 (host), /30 (P2P), /24 (Class C), /16 (Class B), /8 (Class A), and /0 (Default).
- Wildcard mask values — Provided for every prefix, useful for ACL and OSPF configuration.
- Hex representations — Subnet masks are also given as hex octets (e.g., FF.FF.FF.00 for /24).
- Mobile‑friendly design — The table is responsive and can be used on smartphones for field reference.
Who is it for?
- Network engineers — Quickly look up subnet boundaries and host counts when planning IP allocations or troubleshooting routing issues.
- System administrators — Determine correct subnet masks for server and workstation configurations, and validate private IP range assignments.
- IT professionals — Use the wildcard mask column to build ACL rules in routers and firewalls without manual calculation.
- Network architecture designers — Reference classful and classless addressing schemes when designing scalable subnetted networks.
What can you do with it?
- Calculate subnet capacity — For any CIDR prefix, instantly see total IPs and usable hosts (e.g., /23 gives 512 total, 510 usable).
- Convert between notations — Find the dotted‑decimal subnet mask and hex equivalent for any slash notation.
- Identify private vs. public ranges — Use the Private Ranges section to verify that an address falls within RFC 1918 space.
- Configure ACLs — Look up the wildcard mask for a given subnet (e.g., /24 wildcard 0.0.0.255) to match traffic in access‑control entries.
- Understand edge cases — Read the /31 exception explanation for point‑to‑point links where both addresses are host addresses.
FAQ
What is CIDR notation?
CIDR notation is a compact way to represent an IP address and its routing prefix using a slash followed by the number of leading bits (e.g., /24). The Subnet Mask Cheat Sheet uses CIDR notation throughout and provides the equivalent dotted‑decimal subnet mask for each prefix.
Why use /30 for point-to-point links?
A /30 subnet provides 4 total IP addresses: one network, one broadcast, and two usable host addresses. This avoids wasting IP space on point‑to‑point connections where only two devices are needed. The cheat sheet shows /30 as providing 2 usable hosts.
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 with 24 network bits, and 255.255.255.0 is its dotted‑decimal representation. The cheat sheet displays both formats side by side for convenience.
How many usable hosts are in a /23?
A /23 subnet has 512 total IP addresses. Subtracting the network and broadcast addresses leaves 510 usable host addresses. The cheat sheet confirms this instantly.
Yes, the Subnet Mask Cheat Sheet is completely free to access with no registration or login required. All data is displayed on a single page.
