Free tool · China & network
Subnet and CIDR calculator
Work out the address range, netmask and host count of a CIDR block, and how long a transfer takes at a given speed.
IPv4 or IPv6 with a prefix, for example 192.0.2.10/26, 10.0.0.0 255.255.0.0 or 2001:db8::/48.
| Field | Value |
|---|
Transfer time
- Time at full line rate
- –
Protocol overhead and congestion are left out, so real transfers take somewhat longer.
How this works and its limits
Runs in your browser: what you enter stays on your device and is never uploaded.
The calculator works on IPv4 and IPv6 prefixes in your browser: network and broadcast addresses, first and last usable host, netmask and the number of addresses.
Transfer times divide the data size by the line rate and leave out protocol overhead, so real transfers take somewhat longer.
Data sources and licences
Operator names, brands and codes shown here are public reference data compiled from national regulators and other public sources. Their appearance does not indicate any partnership, endorsement, agreement or connectivity between unicomam.com and the named operators. All trademarks belong to their owners. Data is provided “as is” with no warranty; verify against the official source before relying on it for routing, billing or compliance decisions.
Regulatory information is for reference only and is not legal advice; check the regulator link and the “last verified” date.
How CIDR notation works
CIDR notation writes an address block as an address and a prefix length after a slash: the number of leading bits that are fixed for the whole block. The shorter the prefix, the larger the block. An IPv4 /24 fixes 24 of the 32 bits and leaves 8 for hosts, 256 addresses; a /26 leaves 6 bits, 64 addresses.
The prefix is the same thing as a netmask. A /26 is the netmask 255.255.255.192: 26 one-bits followed by zeros. Firewalls and access lists often take the inverse, the wildcard mask, which for a /26 is 0.0.0.63. Every address in the block shares the first 26 bits; the network address has all host bits at zero and, in IPv4, the broadcast address has them all at one.
In an ordinary IPv4 subnet those two addresses are not given to hosts, so a /26 has 62 usable host addresses. Point-to-point links are the exception: a /31 holds exactly the two addresses of the link, and a /32 names a single host. If you enter an address with host bits set, such as 192.0.2.10/26, the calculator shows the block it belongs to, 192.0.2.0/26.
IPv6 works the same way with 128-bit addresses, written in hexadecimal groups with :: standing for a run of zero groups. Blocks are far larger: a single /64, the usual size of one IPv6 subnet, holds 2^64 addresses, and there is no broadcast address. Providers commonly assign a /48 or /56 to a site, which leaves room for many /64 subnets.
Network engineers use CIDR to plan subnets, write firewall rules and describe the address blocks they announce to or accept from a provider. The transfer-time part answers the other everyday question, how long a copy takes: the data size divided by the line rate, before protocol overhead and congestion, which add to the time on any real path.