IPv4 Subnet Calculator

An IPv4 address is 32 bits, usually written as four decimal octets (e.g. 192.168.1.10). A subnet mask (or its shorthand, the /CIDR prefix) tells you how many of those bits identify the network and how many are left over to identify hosts on that network.

Worked example: 192.168.1.10/26

• /26 means the first 26 bits are the network portion, leaving 6 bits for hosts.
• 6 host bits = 2⁶ = 64 total addresses per subnet.
• The subnet mask is 255.255.255.192 (26 ones, then zeros).
• The network address (all host bits zero) is 192.168.1.0.
• The broadcast address (all host bits one) is 192.168.1.63.
• Usable host addresses are everything in between: 192.168.1.1 – 192.168.1.62 (64 − 2 = 62 usable hosts).

Why subnet at all?

Splitting one large network into smaller subnets reduces broadcast traffic, lets you apply different firewall/VLAN policy per segment, and avoids wasting address space on a department that only needs 10 hosts but was handed a /24 (254 usable addresses). That's what the Subnet Splitter and VLSM Planner help you do.

Quick mental model

Bigger prefix number (e.g. /28) → fewer host bits → smaller subnet. Smaller prefix number (e.g. /16) → more host bits → bigger subnet. Every time the prefix grows by 1, the subnet size halves. Full reference: CIDR Cheat Sheet.