Free tool · Subnet calculator
See the mask cut the address.
Network, mask, usable range and broadcast – every calculator does that. This one draws the 32 bits and lets the mask cut them in front of you, and it answers the question operations actually ask: which prefix fits eighty IP phones.
A /26 (255.255.255.192) with 62 usable addresses is enough. · Count printers, access points, phones and cameras – they are almost always missing from the first estimate, and enlarging a network later means touching every device.
Network address
192.168.1.0/24
A private range – these addresses are not reachable from the internet and may be used in any company. With VPN links, make sure the other side does not use the same network: 192.168.1.0/24 on both ends is the most common reason a configured VPN is up and still passes nothing.
Why it matters
Most network faults we see are not faults. They are two subnets that overlap.
A subnet is a decision made once and lived with for years. Made too small, the network fills up with the twentieth printer and every device must be renumbered. Made identical to the one at the other end of a VPN, the tunnel comes up and passes nothing – the single most common VPN ticket we handle. Made from a public range because it happened to be free, it conflicts with the real owner on the internet.
The bit band exists because the formula does not stick, but the picture does. Once you have seen that a /26 leaves six bits for hosts – 64 addresses, 62 usable – you can read any mask. And the reverse slider exists because nobody asks what a /27 is; they ask how many addresses they need for the new site, and then forget the access points.
We operate networks for companies in Germany and host their servers in data centers here. The tool is the one our engineers use before any site goes live; it runs entirely in your browser and sends nothing.
Terms
What the fields mean
- Prefix length (CIDR)
- The number after the slash: how many of the 32 bits belong to the network. /24 means 24 network bits and 8 host bits – 256 addresses, 254 usable.
- Subnet mask
- The same information written as four numbers: 255.255.255.0 is /24. Every 1 bit in the mask is a network bit, every 0 bit a host bit.
- Wildcard mask
- The inverted subnet mask (0.0.0.255 for /24). Used in access lists on some routers and firewalls, where a 0 means 'must match' and a 1 means 'any'.
- Network address
- The first address of the range, all host bits set to 0. It identifies the network and cannot be assigned to a device.
- Broadcast address
- The last address, all host bits set to 1. Packets sent there reach every device in the subnet. Not assignable either – which is why two addresses are lost per subnet.
- Usable addresses
- Total addresses minus network and broadcast. A /30 has four addresses and two usable ones; a /31 (RFC 3021) is the exception for router links and uses both.
- Private ranges (RFC 1918)
- 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are reserved for internal use and never routed on the internet. Everything else is public and uniquely assigned.
Frequently asked questions
Which subnet do I need for 80 devices?
A /25 gives 126 usable addresses, a /26 only 62. Set the device slider to 80 and the calculator names the smallest prefix that fits, including the two addresses every subnet loses to network and broadcast. Count printers, access points, phones and cameras – they are what first estimates forget, and enlarging a subnet later means renumbering every device.
Why does my VPN connect but nothing gets through?
In most cases both ends use the same private network – 192.168.1.0/24 is the factory default of almost every router. The tunnel comes up, but the router cannot tell local from remote because the addresses overlap. One side has to change its subnet; the calculator helps pick a less common one such as 172.16.x.x.
What is the difference between /31 and /30 for a point-to-point link?
A /30 has four addresses, two of them usable – the classic link. A /31 (RFC 3021) has two addresses and uses both, with no network or broadcast address. It halves the address consumption for router-to-router links; most current routers support it, older equipment may not.
Does the calculator handle IPv6?
No, on purpose. An IPv6 calculator is not a bigger IPv4 calculator but a different tool: there the question is never how many addresses remain (there are always enough) but how to split prefixes. Squeezing both into one mask helps neither.
Why are addresses ending in .0 or .255 sometimes usable?
Because network and broadcast are defined by the subnet, not by the last octet. In a /23 (two /24 blocks joined), 192.168.0.255 and 192.168.1.0 are ordinary host addresses; only 192.168.0.0 and 192.168.1.255 are reserved. The bit band shows why: the host bits span more than the last byte.
Sources: RFC 1918 (private ranges), RFC 4632 (CIDR), RFC 3021 (/31 links) – IETF. Read on: RAID calculator · All English tools · Deutsche Fassung