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CIDR to Subnet Mask

Convert CIDR notation to subnet mask and network details. Free online CIDR calculator. No signup, 100% private, browser-based.

CIDR to Subnet Mask

Subnet mask

255.255.255.0

Hosts

254

How it works

CIDR prefix notation (/24, /16, etc.) and dotted-decimal subnet masks (255.255.255.0, 255.255.0.0) are two representations of the same concept: the boundary between the network portion and the host portion of an IP address. Converting between them is a daily task for network engineers, sysadmins, and DevOps engineers configuring firewalls, routing tables, and cloud VPCs.

**The conversion algorithm** CIDR /N → subnet mask: write N ones followed by (32−N) zeros in a 32-bit number, split into four 8-bit octets, convert each to decimal. /24 = 11111111.11111111.11111111.00000000 = 255.255.255.0. /20 = 11111111.11111111.11110000.00000000 = 255.255.240.0. /17 = 11111111.11111111.10000000.00000000 = 255.255.128.0. Reverse: convert each octet to binary, count leading ones.

**Common prefix lengths and their uses** /8 (255.0.0.0): Class A legacy, used for large ISP allocations. /16 (255.255.0.0): typical VPC or campus network block. /24 (255.255.255.0): most common LAN subnet — 254 hosts. /25 (255.255.255.128): splits a /24 into two halves — 126 hosts each. /30 (255.255.255.252): point-to-point links — 2 usable hosts. /32: host route (specific single IP in routing tables).

**Wildcard masks** The wildcard mask (used in Cisco ACLs and OSPF area definitions) is the bitwise inverse of the subnet mask. 255.255.255.0 inverts to 0.0.0.255. Wildcard masks allow non-contiguous patterns, useful for summarizing non-aligned address blocks in access control lists.

Frequently Asked Questions

How do I convert a subnet mask to a CIDR prefix length?
Convert each octet of the subnet mask to binary and count the leading 1 bits. 255.255.255.0 = 11111111.11111111.11111111.00000000 → 24 ones → /24. 255.255.240.0 = 11111111.11111111.11110000.00000000 → 20 ones → /20. A shortcut for powers of 2: 255 = /8 bits, 254 = /7, 252 = /6, 248 = /5, 240 = /4, 224 = /3, 192 = /2, 128 = /1, 0 = /0 (for that specific octet's contribution).
What is a wildcard mask and how does it relate to the subnet mask?
The wildcard mask is the bitwise inverse of the subnet mask: wildcard = 255.255.255.255 − subnet_mask. For 255.255.255.0, wildcard = 0.0.0.255. Wildcard masks are used in Cisco IOS ACLs and OSPF network statements. A 0 bit means 'must match,' a 1 bit means 'any value.' Unlike subnet masks, wildcard masks can be non-contiguous (e.g., 0.0.255.0) for matching non-standard address patterns.
Why do some countries use semicolons in CSV files instead of commas, and does this affect subnet notation?
Subnet masks and CIDR notation are not affected by locale. The semicolon/comma CSV convention relates to decimal separators (Europeans use comma as decimal separator), but IP addresses always use periods as octet separators and CIDR always uses a slash. 192.168.1.0/24 and 255.255.255.0 format is universal regardless of locale settings.
What are the most common subnet masks and when are they used?
/8 (255.0.0.0): large ISP or enterprise allocations, Class A legacy. /16 (255.255.0.0): VPC/campus blocks with up to 65,534 hosts. /24 (255.255.255.0): standard LAN — 254 hosts, most common in enterprise and home networks. /25 (255.255.255.128): splits a /24 into two halves, 126 hosts each. /28 (255.255.255.240): small subnet, 14 hosts — typical for Azure/GCP minimum. /30 (255.255.255.252): point-to-point links, 2 hosts. /32 (255.255.255.255): host route.