Calculators

Three offline calculators for subnetting. No network traffic required — pure maths on the address you type in.

IPv4 subnet calculator

Give it an address and a prefix (CIDR) and it derives every field you'd want for documentation or a firewall rule.

• Address — any IPv4 in dotted notation (10.0.5.7, 192.168.1.100).
• Mask — pick from the dropdown. Shows CIDR next to dotted netmask: /24 (255.255.255.0) down to /1.

Output

• Subnet address — the network's first address (network in older terminology).
• Broadcast — the last address in the range.
• Subnet mask — dotted, e.g. 255.255.255.0.
• Wildcard mask — inverse of the mask (0.0.0.255). What Cisco ACLs want.
• CIDR notation — 192.168.1.0/24.
• Addresses — total, including network and broadcast.
• Hosts — usable count (total − 2).
• First host / Last host — endpoints of the usable range.
• Binary mask — full 32-bit binary expansion.
• IP class — historical class (A/B/C/D/E). Mostly trivia now but still asked in exams.
• IP type — Private / Public / Loopback / Link-local / Multicast / Reserved / Unspecified. Multiple labels if they apply.
• Reverse DNS pointer — the in-addr.arpa name; the PTR record is resolved in the background and the result replaces the placeholder once it arrives.

Splitting a /24

Common task: take 192.168.1.0/24 and break it into four /26s. Enter 192.168.1.0 with mask /26 — the calculator shows the first block. Repeat with 192.168.1.64, 192.168.1.128, 192.168.1.192 to walk the others.

Finding overlap

Two networks overlap if one contains the other. Calculate both with their declared prefixes; if one's first-host ≤ the other's network and one's broadcast ≥ the other's last-host, they collide. Renumber accordingly.

IPv6 calculator

Same idea, scaled up to 128 bits. The address can be entered in any accepted form — compressed (fe80::1), expanded (fe80:0:0:0:0:0:0:1), or with a zone (fe80::1%eth0, parsed but dropped from output).

• Prefix — /4 through /128 in steps of 4. Common picks: /64 (standard LAN), /56 (customer site), /48 (organisation), /128 (single host).
• Address — both compressed and fully exploded forms side-by-side.
• First / last address in the range, compressed.
• Hosts — 2^N for N ≥ 64, formatted number for smaller prefixes. A /64 holds 2^64 ≈ 1.8·10^19 addresses.
• Number of /64s — how many standard LANs fit in the prefix (/48 = 65 536 /64s, /56 = 256).

Common IPv6 blocks

Prefix	Purpose
fe80::/10	Link-local; auto-configured on every interface.
fc00::/7	Unique local (ULA); IPv6 equivalent of RFC1918.
2000::/3	Global unicast.
ff00::/8	Multicast.
::1/128	Loopback.
2001:db8::/32	Documentation (safe to use in examples).

IPv6 subnet splitter

Takes a parent prefix and a desired number of subnets, then produces them all with their boundaries. Does the work the single-subnet calculator won't.

• Address — the parent block (e.g. 2001:db8:1234::).
• Prefix — the parent's prefix length (e.g. /48).
• Number of subnets — how many you need. The tool picks the smallest host-bits that fit that count.

Output per subnet

• Index (1…N) and the chosen child prefix (e.g. /52 if you asked for 8 subnets out of a /48).
• Compressed and exploded address.
• Binary view split into network bits, subnet bits, host space — colour-coded so you see the split visually.
• Last address in the subnet.
• Host count — 2^N notation for large subnets.

Subnets beyond the requested count are shown greyed out (excess from rounding up to a power of two) — pick a nice round number like 4, 8, 16 to avoid leftovers.

Example: carving a /48 into site blocks

You have 2001:db8:abcd::/48 and want 16 sites, each with its own /52. Enter address 2001:db8:abcd::, prefix /48, subnets 16. The result is sixteen /52 blocks (2001:db8:abcd:0::, ...:1000::, ...:2000::, …). Each of those still has room for 4096 /64 LANs.