IPv4 Subnet Calculator
Processed Client SideEnter an IPv4 address with a CIDR prefix or a subnet mask to get the network and broadcast addresses, the usable host range, the wildcard mask, and a bit-level view of where the mask cuts.
Bookmark this tool now — skip the search next time you need it.
About IPv4 Subnet Calculator
This tool runs entirely in your browser. Whatever you paste is processed on your own device and is never uploaded, logged, or sent to any server.
The IPv4 Subnet Calculator turns a CIDR block into every number you need to configure a network: the network and broadcast addresses, the first and last usable host, how many hosts you get, and the subnet and wildcard masks in both dotted-decimal and binary. Feed it an address with a prefix, a dotted mask, or neither, and it also splits the block into equal smaller subnets so you can plan an allocation without binary arithmetic on paper. A prefix slider resizes the network live, the binary view colours network bits apart from host bits so you can see where the mask cuts, and reserved ranges such as RFC 1918 private space or carrier-grade NAT are named as soon as you paste an address inside one.
Key features
- Accepts every common notation — 192.168.1.10/24, 10.0.0.5 255.255.255.0, 172.16.5.4/255.255.240.0, or a bare address treated as /32
- Network address, broadcast address, first and last usable host, and the full host range
- Total address count and usable host count, with /31 and /32 handled correctly under RFC 3021
- Subnet mask, wildcard mask (the ACL form Cisco wants), prefix length, and the network/host bit split
- Binary breakdown of the address, mask, network and broadcast, with the network bits and host bits coloured differently so you can see exactly where the mask cuts
- Scope detection — RFC 1918 private, loopback, link-local/APIPA, carrier-grade NAT, documentation, multicast, and reserved ranges are all named
- Legacy class (A/B/C/D/E), integer and hexadecimal forms, and the in-addr.arpa name for reverse DNS
- Split a block into equal subnets at any longer prefix and get a table of every resulting network with its host range
- A prefix slider that recalculates live, so you can feel the trade-off between subnet size and host count
- Runs entirely in your browser — nothing is uploaded
How to use it
- Type or paste an IPv4 address with its prefix into the left pane, for example 192.168.1.10/24.
- Read the network details on the right — addresses, masks, counts, and the binary view.
- Drag the prefix slider, or pick a prefix from the dropdown, to see how the network resizes.
- To plan an allocation, choose a longer prefix in "Split into" and read the table of resulting subnets.
- Copy the whole report with the Copy button, or copy any single value with the icon beside it.
Tips & common mistakes
- Usable hosts is always two fewer than total addresses for /30 and shorter: the first address identifies the network and the last is the broadcast address. A /24 gives 256 addresses but only 254 hosts.
- A /31 is the exception. RFC 3021 makes both addresses usable on point-to-point links, which is why the calculator reports 2 usable hosts rather than 0.
- The wildcard mask is the inverse of the subnet mask, and it is what Cisco ACLs and OSPF network statements expect — /24 is mask 255.255.255.0 but wildcard 0.0.0.255.
- If you enter a host address rather than a network address, the network row shows the block it belongs to. 192.168.1.10/24 and 192.168.1.200/24 are the same network.
- A dotted mask must be contiguous — all ones then all zeros. 255.255.0.255 is a valid-looking quad but not a valid mask, and the tool rejects it rather than guessing.
- To halve a network, add one to the prefix; each extra bit doubles the number of subnets and halves the hosts in each. Splitting a /24 into /26s gives four subnets of 62 hosts.
- The classful A/B/C labels are historical. Routing has been classless since CIDR arrived in 1993 — the prefix, not the first octet, decides the network size.