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CIDR / subnet calculator

Paste a prefix such as 10.0.0.0/28 or 2001:db8::/32. You can also type an IPv4 address and a dotted netmask.

Address or CIDR

Result

Is this address in the subnet?

IPv4 address

Split into smaller IPv4 subnets

New prefix length Must be longer than the current prefix. Lists up to 256 subnets.

How to size a network on a small VPS

/32 is one host, which is what you usually assign to the VPS itself. /31 is a point-to-point link (RFC 3021) with two usable addresses and no broadcast. /30 is the older four-address point-to-point default, of which only two are usable. /29 gives six usable hosts, often enough for a tiny lab; /28 gives fourteen.

When you add Docker, LXC, or WireGuard, pick a range that does not collide with the host, with RFC 1918 networks you already use at home, or with CGNAT 100.64.0.0/10. This calculator flags those special-use ranges so you can see the clash before you type ip addr add.

Class A/B/C labels are historical. Routing today is CIDR. IPv6 math is included for prefix length and first/last address; splitting and “contains” checks are IPv4-only in this version.