How to read the CIDR table
The interactive CIDR table is a reference for IPv4 subnetting. Each row connects a prefix such as /24, /26 or /30 with its dotted-decimal mask, remaining host bits, block size, host capacity and subnet increment. Instead of memorizing isolated values, use the table to see how all of those properties change together.
A CIDR prefix tells you how many of IPv4's 32 bits belong to the network portion. The remaining bits determine the number of addresses in the block. The subnetting guide explains the full calculation.
Prefix length and dotted-decimal mask
A /24 corresponds to 255.255.255.0, a /25 to 255.255.255.128, a /26 to 255.255.255.192 and so on. Valid IPv4 subnet masks contain contiguous one bits followed by contiguous zero bits, which is why every valid mask maps to exactly one CIDR prefix.
If you need to convert one value directly, use the subnet mask ↔ CIDR converter. The table is better for comparing many prefixes at once.
Host bits, block size and capacity
The number of host bits is 32 - prefix. The total block size is therefore 2^(host bits). A /24 has 8 host bits and 256 addresses. A /26 has 6 host bits and 64 addresses. A /30 has 2 host bits and 4 addresses.
The table lets you switch host semantics. Traditional LAN mode shows the classic capacity after reserving network and broadcast where applicable. Point-to-point semantics account for /31 usage under RFC 3021, and exact mode displays the total number of addresses without applying host-reservation assumptions.
Understanding subnet increment
The subnet increment, sometimes called block step, is a practical shortcut for finding boundaries in the octet where the mask changes. With 255.255.255.192, or /26, the fourth octet advances by 64: 0, 64, 128 and 192. With /27 it advances by 32.
The increment does not replace binary understanding, but it makes manual subnetting faster. The subnet block-size calculator focuses specifically on that relationship.
Common prefixes worth recognizing
For day-to-day IPv4 work, prefixes around /16 through /30 appear frequently. A /24 is a 256-address block, /25 has 128, /26 has 64, /27 has 32, /28 has 16, /29 has 8 and /30 has 4. Recognizing this halving pattern is more useful than memorizing every row independently.
The interactive filters allow you to focus on common prefixes or search directly for a prefix, mask, block size or host count. This makes the table useful both as a study aid and as a quick validation reference.
Special cases: /31 and /32
A /31 contains two addresses and can be used on IPv4 point-to-point links under RFC 3021. A /32 represents one address or host route. Applying “block size minus two” blindly would give misleading results for these prefixes, so the host-mode selector exposes the semantic difference explicitly.
When designing a traditional LAN, keep those special cases separate from ordinary multi-host subnets. The CIDR calculator lets you experiment with host requirements and semantics interactively.
Frequently asked questions
Why does increasing the prefix make the block smaller?
Each additional network bit leaves one fewer host bit, so the number of addresses is halved.
Is /24 always 255.255.255.0?
Yes. CIDR prefix length and a valid contiguous subnet mask are two notations for the same bit boundary.
Why can /31 have two usable point-to-point addresses?
RFC 3021 removes the traditional network/broadcast interpretation for IPv4 point-to-point /31 links.
Should I memorize every mask?
No. Learn the binary and halving pattern, then use the table as a reference while the common values become familiar.
Use the table as a validation tool
When solving subnetting manually, calculate the prefix, mask, capacity and increment yourself first, then compare the result with the table. That workflow helps you build intuition without becoming dependent on a lookup.