How to use
In “Subnet calculator”, enter something like 192.168.1.10/24 (a mask such as “10.0.0.1 255.255.255.0” also works).
See the network, broadcast, host range, host count and masks. Move the slider to change the prefix length instantly.
To divide a network, use “Split · VLSM”: equal-size subnets, or enter the hosts each team needs. Save the plan as CSV.
In “Range ⇔ CIDR · Summarise”, convert a start–end range to CIDRs or merge a messy list into the minimal CIDR set.
In “Check IP list”, test many IP addresses against a list of networks such as an allow-list.
What it does
IPv4 and IPv6
IPv6 is shown in RFC 5952 compressed and fully expanded forms, and huge counts such as a /64 are exact (shown as 2^64). Zone IDs (%eth0) and IPv4-mapped addresses are understood.
See the network bits
Address, mask and network are shown in binary with network and host bits coloured. You are told when the address entered is not the network address. /31 point-to-point and /32 are handled correctly.
VLSM planning with CSV export
List the hosts each subnet needs; subnets are allocated largest first on block boundaries, with anything that does not fit and the unused blocks shown. Equal splits list up to 4,096 subnets.
Range ⇔ CIDR and summarisation
Convert ranges like “192.168.1.10 - 192.168.1.20” into the minimal CIDR list, and merge overlapping or adjacent networks into the fewest CIDRs — handy for tidying firewall rules.
Address type at a glance
Private, CGNAT, link-local, documentation, multicast, ULA and other IANA special-purpose ranges are labelled with their RFC.
Use cases
- Check the network range and usable IPs before configuring a router, AWS VPC or server
- Design subnets for each team or floor from the number of hosts they need (VLSM)
- Turn IP ranges into CIDRs or wildcard masks for firewall rules and ACLs
- Check whether IPs from access logs are inside the office network or an allow-list
- Verify answers when studying for networking exams such as CCNA
Capability matrix
| Feature | Support | Notes |
|---|---|---|
| IPv4 / IPv6 subnet calculation | Supported | — |
| Input with subnet or wildcard masks | Supported | — |
| Equal split, VLSM planning, CSV export | Supported | Lists show up to 4,096 rows |
| Range ⇔ CIDR conversion and summarisation | Supported | — |
| Check, sort and de-duplicate IP lists | Supported | Up to 10,000 IPs per check |
| Reverse DNS zone names (in-addr.arpa / ip6.arpa) | Supported | — |
| Show my public IP, WHOIS, geolocation lookups | Not supported | No external lookups are made |
Limits & notes
- IPv4 addresses are four numbers 0–255. Numbers with leading zeros (such as 010) are rejected because some devices read them as octal.
- Equal splits list the first 4,096 subnets and range conversion returns up to 4,096 CIDRs. Up to 10,000 IPs can be checked at once.
- VLSM host counts for IPv4 exclude the network and broadcast addresses (/31 and /32 are not used for allocation).
- There is no lookup of who owns an IP or where it is (WHOIS / geolocation).
Privacy & safety
The IP addresses and lists you enter are calculated in your browser only. Nothing is sent to or stored on a server.
FAQ
What is the subnet mask of a /24 and how many hosts does it have?
How are the network and broadcast addresses calculated?
What is a wildcard mask?
What are the private IP ranges?
Are the IP addresses I enter sent anywhere?
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