IPv4 Subnet Calculator & CIDR Cheat Sheet

Quick reference

CIDR subnet cheat sheet: /24 to /31

Use this table to check the most common small IPv4 subnets. Traditional usable-host counts exclude the network and broadcast addresses; /31 is the point-to-point exception described in RFC 3021.

Inverse masks

What is a wildcard mask?

A wildcard mask is the bitwise inverse of a subnet mask. In access-control lists and routing configuration, a 0 bit means the corresponding address bit must match; a 1 bit means it may vary.

Calculate it one octet at a time by subtracting the subnet-mask octet from 255. A /24 mask of 255.255.255.0 becomes 0.0.0.255. A /26 mask of 255.255.255.192 becomes 0.0.0.63, and a /30 becomes 0.0.0.3.

Worked calculation

Calculate 192.168.10.77/26 step by step

A worked example makes the boundary logic easier to verify than memorising a table. For 192.168.10.77/26, only the final octet changes within the subnet.

1. 1. Convert /26 to a subnet mask

   Twenty-six network bits produce 255.255.255.192.

2. 2. Find the block size

   256 − 192 = 64, so the last-octet subnet boundaries are 0, 64, 128, and 192.

3. 3. Locate the IP address

   The final octet 77 falls inside the 64–127 block.

4. 4. Identify network and broadcast

   The network address is 192.168.10.64 and the broadcast address is 192.168.10.127.

5. 5. Identify hosts and wildcard

   Traditional usable hosts run from 192.168.10.65 through 192.168.10.126. The wildcard mask is 0.0.0.63.

Network planning

Where these subnet results are useful

Subnetting is practical address planning, not just certification math. Use the calculator to check a design before applying it, and confirm production changes against the platform documentation.

• Plan a /24 LAN with up to 254 traditional host addresses for clients, printers, phones, and infrastructure.
• Split a /24 into two /25 networks to separate departments, floors, or security zones.
• Use /28, /29, or /30 blocks where a small fixed number of addresses is needed.
• Use /31 on supported point-to-point links to avoid wasting network and broadcast addresses.
• Generate wildcard masks for router or firewall ACLs that use inverse-mask matching.
• Check DHCP scope boundaries so the pool stays inside the intended network and avoids reserved infrastructure addresses.

Before deployment

Common subnetting mistakes

A calculator prevents arithmetic slips, but the result still has to be applied correctly. These are the mistakes worth checking before changing a live network.

• Reading /24 as “24 hosts” instead of 24 network-prefix bits.
• Using the total address count as the traditional usable-host count without removing network and broadcast addresses.
• Entering a wildcard mask where the device expects a subnet mask, or the reverse.
• Assigning the network or broadcast address to a host on a traditional /30-or-larger subnet.
• Treating /31 like a traditional subnet instead of checking whether the point-to-point devices support RFC 3021.
• Forgetting that overlapping subnets can create routing ambiguity even when each individual mask is valid.