How to use this tool
- Enter an address with a slash prefix, such as 192.168.1.42/24 or 2001:db8::/64.
- Optionally enter an address to test against the range.
- Calculate the canonical network, first and last address, exact count and membership result.
What does the slash number mean?
CIDR describes a block using a network address and a prefix length. IPv4 has 32 address bits, so valid prefixes run from /0 to /32. IPv6 has 128 bits, so valid prefixes run from /0 to /128. A longer prefix describes a smaller block.
If you enter an address in the middle of a block, the tool clears its host bits to show the actual network. For example, 192.168.1.42/24 becomes 192.168.1.0/24. The boundaries include all addresses in the block; this tool does not subtract IPv4 network or broadcast addresses.
IPv6 counts need exact arithmetic
A /64 contains 18,446,744,073,709,551,616 IPv6 addresses. Ordinary floating-point numbers cannot represent every integer at that scale. The calculator uses integer arithmetic and displays the full count without rounding.
IPv6 has no IPv4-style broadcast address. The last address is simply the high boundary of the block. The tool does not generate a list of hosts, scan the range or infer which addresses are assigned.
Membership keeps families separate
An IPv4 address cannot belong to an IPv6 range, even when an IPv6 representation embeds IPv4. A same-family membership test compares the address with the computed low and high boundaries. It does not prove the host is on your local network or reachable.
Use the IPv4 subnet calculator when you need a dotted mask, broadcast and conventional usable-host range. Use the IPv6 formatter when you want to expand or compress a single IPv6 address without changing its value.