Subnet Calculator

This calculator converts an IPv4 CIDR prefix (the number after the slash, such as /24) into its dotted-decimal subnet mask, the total number of addresses in the block and how many of those you can assign to devices. Enter how many hosts you need and it also tells you the longest prefix — the smallest subnet — that still fits them.

Network engineers use it when carving a VLAN or VPC address space into subnets, sizing a DHCP scope, or reading a firewall rule written in CIDR notation. Each step of one in the prefix halves the block, so picking the wrong prefix either wastes addresses or leaves no room for growth.

The tool covers IPv4 only. Masks are just binary numbers written in four 8-bit groups, so a binary converter or base converter helps if you want to see exactly which bits are network bits.

CIDR notation is defined in IETF RFC 4632. The −2 rule applies to ordinary subnets from /0 to /30. Two prefixes are exceptions: a /31 has two addresses that RFC 3021 allows both to be used on point-to-point router links, and a /32 identifies a single host (common in routes and firewall rules). This calculator reports 0 usable hosts for both, which is correct for normal LAN use.

This page also replaces the former IP range, IP address count and subnet CIDR calculators: the section below shows how to get the first and last address of any block from its size.

Each extra prefix bit halves the block. Within the last octet, the mask value is always 256 minus the block size: a 64-address block has a last octet of 256 − 64 = 192.

Subnets always start on a multiple of their block size, which makes the address range easy to work out by hand:

• Block size = total addresses from the calculator (for a /26, 64).
• Network address: round the relevant octet of the IP down to a multiple of the block size. For 192.168.10.77/26, 77 rounds down to 64, so the network is 192.168.10.64.
• Broadcast address: network + block size − 1, here 192.168.10.127.
• Usable range: everything in between, 192.168.10.65 to 192.168.10.126 (62 hosts).

For prefixes shorter than /24 do the same in the third octet: 10.20.37.5/20 has a block of 16 in the third octet, so the range runs from 10.20.32.0 to 10.20.47.255.

RFC 1918 sets aside three blocks for private networks: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Home routers usually hand out a /24 from 192.168.x.x, while companies and cloud VPCs typically carve subnets out of 10.0.0.0/8.

To split a block, borrow bits: a /24 divides into two /25s, four /26s or eight /27s. A larger parent block holds 2^(child − parent) subnets — a /16 contains 256 separate /24s.