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A complete engineer's cheat sheet for IPv4 Classless Inter-Domain Routing (CIDR). Includes dotted decimal subnet masks, wildcard masks, and host counts from /8 down to /32.
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Network engineers, cloud architects, and sysadmins frequently need to convert CIDR notation into dotted decimal masks and host capacities without manual binary conversion.
Below is the complete reference table. Use our Subnet Calculator to instantly calculate usable host ranges, broadcast addresses, and binary bitmasks for any IP block.
CIDR Subnet Mask Wildcard Mask Total IPs Usable Hosts Typical Use Case /8 255.0.0.0 0.255.255.255 16,777,216 16,777,214 Class A Private / ISP Backbone /16 255.255.0.0 0.0.255.255 65,536 65,534 Class B Private / Large AWS VPC /20 255.255.240.0 0.0.15.255 4,096 4,094 Corporate Campus / Kubernetes /24 255.255.255.0 0.0.0.255 256 254 Standard LAN / Office VLAN /25 255.255.255.128 0.0.0.127 128 126 Subnet Partition (1/2 /24) /26 255.255.255.192 0.0.0.63 64 62 Mid Server Cluster (1/4 /24) /27 255.255.255.224 0.0.0.31 32 30 Branch Office or Staging Zone /28 255.255.255.240 0.0.15 16 14 Database Cluster / Small DMZ /29 255.255.255.248 0.0.0.7 8 6 Public IP Allocation / Firewall HA /30 255.255.255.252 0.0.0.3 4 2 Legacy Point-to-Point WAN Link /31 255.255.255.254 0.0.0.1 2 2 RFC 3021 Modern P2P Router Link /32 255.255.255.255 0.0.0.0 1 1 Single Host / Loopback Route
The formula to determine the total number of IP addresses in an IPv4 prefix is 2^(32 - Prefix).
For standard subnets (/1 through /30), subtract 2 addresses: the first address is reserved as the Network Identifier, and the last address is reserved as the Subnet Broadcast Address.
Example for /27: 32 - 27 = 5 host bits. 2^5 = 32 total addresses. 32 - 2 = 30 usable host IP addresses.
Historically, /30 subnets were used for router-to-router point-to-point links, consuming 4 IP addresses to provide only 2 usable endpoints (50% address waste).
RFC 3021 introduced support for /31 prefixes on point-to-point links. In a /31 subnet, the network and broadcast addresses are eliminated, making both addresses usable as host endpoints and conserving valuable IPv4 address space.
AI Mobile App Developer & Full-Stack Systems Engineer•Dhaka, Bangladesh
Bayajit Islam is an AI Mobile App Developer and Full-Stack Systems Engineer based in Dhaka, Bangladesh. With 2+ years of hands-on production experience building cross-platform apps for iOS, Android & Desktop with Flutter & Dart, asynchronous backends with FastAPI & Python, and modern web applications with Next.js, TypeScript, and Docker, he engineered LotsofNetwork as a zero-latency, privacy-first diagnostic platform for developers and network administrators.
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