Tool 2.11

Subnet calculator for IPv4 and IPv6 (CIDR)

Type an IP address with its prefix to get the network, mask, broadcast, usable host range and count, and split it into equal subnets. IPv4 and IPv6.

Runs on your deviceRuns entirely in your browser: nothing you type or upload leaves your device.

How to use it

Type an address with its prefix, like `10.0.1.0/24` or `2001:db8::/48`. For IPv4 you can write the mask instead: `10.0.1.0 255.255.255.0`.
Read the network, mask, broadcast, first and last usable host and the number of addresses. The results update as you type.
Under Split into equal subnets, choose 2, 4, 8 or 16 parts, then copy the summary or the table.

What the calculator gives you, and when you need it

Subnetting means cutting a block of IP addresses into smaller networks. You need it when you set up a router, keep the office computers apart from the security cameras, give each branch its own range over a VPN, or fill in the network fields of a cloud server. Each of those asks for a network address and a prefix, and a mistake leaves two networks that overlap or a gateway outside its own range.

Type any address in the block with its prefix length, the number after the slash. The tool works out the network it belongs to and shows the subnet mask, the wildcard mask (the mask inverted, as Cisco access lists and OSPF use it), the broadcast address, the first and last usable host, the number of usable hosts and the total number of addresses. It also says what kind of block it is: private (RFC 1918), shared space for carrier NAT, loopback, link-local, documentation or public. Everything is calculated in your browser, and nothing is sent.

Example: 10.0.10.77/26

A /26 keeps 26 bits for the network and leaves 6 for hosts, so the block has 2^6 = 64 addresses. The tool shows the network 10.0.10.64/26, the mask 255.255.255.192, the broadcast 10.0.10.127 and usable hosts from 10.0.10.65 to 10.0.10.126, 62 in all. In IPv4 the first address names the network and the last one is the broadcast, so neither goes to a device.

Split 10.0.10.0/24 into 4 and you get four /26 networks that start at .0, .64, .128 and .192, with 62 usable hosts each: one range for staff, one for guest Wi-Fi, one for cameras and printers, and one kept free for later.

Special cases and what the tool can't see

On a point-to-point link between two routers, RFC 3021 allows a /31: both addresses are hosts and there is no broadcast, so the tool shows 2 usable hosts. A /32 is a single address, as used for loopbacks and firewall rules.

IPv6 has no broadcast address. The tool shows the first and last address and counts every address in the range. A /64 is the usual size for one network segment, with 18,446,744,073,709,551,616 addresses, and SLAAC, the way devices give themselves an IPv6 address, needs it. Addresses are shown in the short form of RFC 5952 and also written out in full.

The calculator only does the arithmetic. It can't tell whether an address is in use, or whether two ranges overlap in your router. IPv4 addresses with leading zeros, such as 010.0.0.1, are refused, because some programs read them as octal numbers.

FAQ

What does /24 mean?

It's the prefix length: the first 24 of the 32 bits of an IPv4 address identify the network, and the other 8 identify devices. /24 is the same as the mask `255.255.255.0` and gives 256 addresses, 254 of them usable.

How many usable hosts does a /30 have?

Two. A /30 has 4 addresses: the network, two hosts and the broadcast. It was the classic size for a link between two routers; a /31 does the same job with 2 addresses (RFC 3021).

Is 100.64.x.x a private address?

It's shared address space (RFC 6598), set aside for internet providers that put many customers behind one public address, known as carrier-grade NAT. It isn't one of the RFC 1918 private ranges, but it isn't reachable from the internet either. If your router's WAN address is in `100.64.0.0/10`, you are behind your provider's NAT, and forwarding a port on your router won't make a service reachable from outside.

Can I type a subnet mask instead of a prefix?

Yes, for IPv4: `10.1.2.3 255.255.0.0` or `10.1.2.3/255.255.0.0`. The mask must be ones followed by zeros; `255.0.255.0` is refused, because no prefix can express it.

Why does the tool say the block is mixed?

The block you typed is larger than one of the special ranges and contains part of it. For example, `10.0.0.0/7` covers `10.0.0.0/8`, which is private, and `11.0.0.0/8`, which is public.

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