geeksubnet — free online IP subnet calculator

What you can calculate

geeksubnet bundles thirteen networking tools into one terminal-style interface. Every result is computed locally in JavaScript, so the IP ranges you enter never leave your machine — handy when you are planning addressing for a client network or documenting infrastructure you would rather not paste into a third-party site.

• IPv4 subnet calculator — enter an address and a CIDR prefix (for example 192.168.1.42/24) and instantly see the network address, broadcast address, subnet mask, wildcard mask, usable host range, total and usable host counts, address class and a live binary view of the mask.
• IPv6 calculator — expand or compress any IPv6 address, derive the network prefix, and identify whether it is global unicast, link-local, unique local (ULA) or multicast.
• IP converter — translate an address between dotted-decimal, 32-bit integer, hexadecimal and binary representations.
• Hosts → prefix — tell it how many usable hosts you need and it returns the smallest CIDR prefix that fits them, with the matching mask and total address count.
• Subnet membership — check whether a given IP address falls inside a given network, with all the surrounding subnet details.
• Range → CIDR — convert an arbitrary start–end address range into the minimal list of CIDR blocks that covers it exactly. Indispensable for firewall rules, ACLs and BGP prefix lists.
• VLSM solver — give it a parent network and a list of departments with different host requirements; it allocates correctly sized, non-overlapping subnets and reports the allocation efficiency.
• Subnet splitter — divide one network into 2^k equal sub-networks in a single step.
• Supernet aggregator — find the smallest single CIDR that contains a list of networks, the way route summarisation works in OSPF and BGP.
• Ports browser — a searchable copy of the IANA service-name and port-number registry, filterable by TCP, UDP, SCTP or DCCP.
• MAC tools — look up the vendor (OUI) of a MAC address, derive its EUI-64 interface identifier, and inspect the IEEE-administered bits.
• MTU / MSS calculator — work out the effective payload left after IPv4/IPv6, TCP or UDP, and tunnel overhead such as VLAN, PPPoE, GRE, IPsec, WireGuard or VXLAN.
• Quiz mode — practise CIDR and subnetting under time pressure, with a score that persists between visits.

Each tool has a one-click export to .txt or .csv, so you can drop a subnet plan straight into documentation or a spreadsheet.

Subnetting in sixty seconds

An IPv4 address is 32 bits, usually written as four decimal octets such as 192.168.1.42. A subnet mask splits those 32 bits into a network part and a host part. In CIDR notation the mask is written as a slash followed by the number of network bits — /24 means the first 24 bits identify the network and the remaining 8 bits identify hosts.

Network and broadcast addresses

The network address is the first address in a subnet: all host bits set to zero. The broadcast address is the last: all host bits set to one. Neither can be assigned to a device, which is why a /24 with 256 total addresses offers 254 usable hosts.

Counting hosts

A prefix of length n leaves 32 − n host bits, giving 2⁽³²⁻ⁿ⁾ total addresses. Subtract two for the network and broadcast addresses to get the usable count. A /26 therefore has 64 total addresses and 62 usable hosts. The two exceptions are /31, which RFC 3021 defines for two-host point-to-point links, and /32, a single host route.

Wildcard masks

A wildcard mask is the bitwise inverse of the subnet mask. Cisco access control lists use it: the wildcard for /24 is 0.0.0.255.

A worked example: planning a /24 for four teams

Suppose you own 192.168.10.0/24 and need to carve it for four departments needing 50, 25, 10 and 10 usable hosts. Sorting largest first, the 50-host team needs a /26 (62 usable), the 25-host team a /27 (30 usable), and the two 10-host teams a /28 each (14 usable):

192.168.10.0/24
├── 192.168.10.0/26     50 hosts  →  192.168.10.1   – 192.168.10.62
├── 192.168.10.64/27    25 hosts  →  192.168.10.65  – 192.168.10.94
├── 192.168.10.96/28    10 hosts  →  192.168.10.97  – 192.168.10.110
└── 192.168.10.112/28   10 hosts  →  192.168.10.113 – 192.168.10.126

That uses 128 of the 256 addresses, leaving 192.168.10.128/25 free for growth. The VLSM solver above produces exactly this layout, including the leftover space, and the subnet splitter will divide any block into equal pieces when every team needs the same size.

CIDR quick reference

Frequently asked questions

Is geeksubnet free to use?

Yes. Every tool is free, requires no account, and runs without limits.

Do you store or send the IP addresses I enter?

No. All calculation happens in your browser with JavaScript. The addresses, ranges and MAC values you type are never transmitted to any server.

Does it work offline?

Once the page has loaded, the core calculators run with no network connection. The ports and MAC-vendor lookups load their datasets the first time you open them.

What is the difference between a /30 and a /31?

A /30 provides four addresses with two usable hosts plus a network and broadcast address. A /31 provides two addresses, both usable, defined by RFC 3021 specifically for two-host point-to-point links to save address space.

How many usable hosts are in a /22?

A /22 has 10 host bits, which is 1,024 total addresses. Subtract the network and broadcast addresses and you get 1,022 usable hosts.

Can it convert an IP address range into CIDR blocks?

Yes — the range-to-CIDR tool turns any start–end range into the smallest set of aligned CIDR blocks that covers it exactly, which is what you need for firewall and ACL rules.

Does it support IPv6?

Yes. The IPv6 calculator expands and compresses addresses, derives prefixes, and classifies global, link-local, unique-local and multicast ranges. The MAC tools also derive EUI-64 interface identifiers used by SLAAC.

In-depth guides

Beyond this overview, the networking guides walk through the trickier topics with full worked examples you can verify in the calculator:

• Subnetting basics — the binary you actually need, and how to split a /24 by hand.
• VLSM design — the allocate-largest-first method with a five-segment worked plan.
• IPv6 for IPv4 engineers — hex, compression, prefix types, SLAAC and EUI-64.
• Subnet mask cheat sheet — every prefix /8–/32 plus the magic-number trick.