IPv6 Subnet Calculator
Compute the network prefix, address range, and total addresses for any IPv6 subnet.
Inputs
Full or compressed form, e.g. 2001:db8::1
0–128. /64 is the typical LAN prefix.
Network / Prefix
2001:db8:85a3::/64
Compressed Address
2001:db8:85a3::8a2e:370:7334
Full (Expanded) Address
2001:0db8:85a3:0000:0000:8a2e:0370:7334
First Address in Block
2001:db8:85a3::
Last Address in Block
2001:db8:85a3:0:ffff:ffff:ffff:ffff
Total Addresses (power)
2^64
Total Addresses (count)
18,446,744,073,709,551,616
Address Type
Global Unicast
Step by step
Expand the address to all 128 bits
2001:db8:85a3::8a2e:370:7334
= 2001:0db8:85a3:0000:0000:8a2e:0370:7334
Any '::' run is restored to the zero groups it stands for.
Split at the /64 boundary
64 network bits, 64 host bits
= mask = 64 leading 1-bits
Mask off the host bits to get the network
2001:db8:85a3::8a2e:370:7334 AND /64 mask
= 2001:db8:85a3::
Set all host bits to get the last address
2001:db8:85a3:: OR 64 low bits
= 2001:db8:85a3:0:ffff:ffff:ffff:ffff
Count the addresses in the block
2^(128 − 64) = 2^64
= 18,446,744,073,709,551,616
IPv6 blocks are vast by design — a /64 is the standard size for a single subnet.
How it works
An IPv6 address is 128 bits, usually written as eight groups of four hex digits. The prefix length says how many leading bits identify the network. Applying the prefix mask gives the network address; the remaining host bits define the range from the first to the last address, for 2^(128 − prefix) total addresses. This calculator also shows the fully-expanded and RFC 5952-compressed forms.
Formulas
Network address
network = ip AND mask, where mask is the top `prefix` bits set
- ip
- The 128-bit address
- mask
- The prefix mask — `prefix` leading 1-bits, the rest zero
- prefix
- Prefix length, 0–128
Address count
count = 2^(128 − prefix)
- 128 − prefix
- Host bits — the part of the address free to vary
Guides that use this calculator
- How to Calculate a Subnet Mask (By Hand and Quickly)Work out subnet masks, network and broadcast addresses and host counts by hand — with the binary shortcuts network engineers actually use.8 min
- What Is CIDR Notation, and Why Did It Replace Network Classes?CIDR explained from first principles: what the slash number means, why classes A/B/C were abandoned, and how to read any prefix at a glance.7 min
Frequently Asked Questions
Why is a /64 the standard subnet size?
IPv6 was designed so each LAN gets a /64, leaving 64 host bits. That's 18.4 quintillion addresses per subnet, which supports stateless address autoconfiguration (SLAAC).
How does '::' compression work?
A single '::' replaces the longest run of consecutive all-zero groups (at least two). It can appear only once in an address. This tool shows both the compressed and fully-expanded forms.
Does IPv6 have broadcast or network/host reserved addresses?
No. IPv6 has no broadcast address, and unlike IPv4 it doesn't reserve the first and last addresses of a subnet, so all 2^(128 − prefix) addresses in the block are usable in principle.