IPv6 Subnetting Explained
IPv6 subnetting is simpler than IPv4 in one way: the subnet is almost always a /64. You subnet by borrowing bits between the site prefix (typically /48) and /64 — and the easiest place to do it is on nibble (4-bit) boundaries.
The standard allocations
IPv6 addresses are 128 bits, written as eight groups of four hex digits. The common hierarchy:
| Allocation | Prefix | What it means |
|---|---|---|
| ISP from RIR | /32 | A large block an ISP receives |
| Site / customer | /48 | Typical assignment to one organisation |
| Subnet (LAN) | /64 | One subnet — the standard size |
| Point-to-point | /127 | Router links (RFC 6164) |
A /64 is the standard LAN subnet because SLAAC (stateless auto-configuration) needs 64 host bits.
How many subnets does a /48 give?
From /48 to /64 you have 64 − 48 = 16 subnet bits. That is 2^16 = 65,536 /64 subnets — far more than any site needs, which is the point.
2001:db8:acad::/48 subnet 0000 → 2001:db8:acad:0000::/64 subnet 0001 → 2001:db8:acad:0001::/64 subnet 000a → 2001:db8:acad:000a::/64 ...up to ffff
Nibble boundaries
Each hex digit is 4 bits (a "nibble"). Subnetting on nibble boundaries (/52, /56, /60, /64) keeps the maths in hex trivial because you change one hex digit at a time. A /56, for example, gives 2^8 = 256 /64 subnets and is a common home/branch allocation.
How IPv6 differs from IPv4 subnetting
- No broadcast address — IPv6 has no broadcast, so you do not subtract 2. Every address in a /64 except a few reserved ones is usable.
- Subnet size is fixed at /64 for LANs — you rarely change host-bit counts the way you do in IPv4 VLSM.
- Hex, not dotted decimal — you count subnets in hexadecimal.
See also IPv4 vs IPv6 for the wider comparison.
Frequently asked questions
Why is /64 the standard IPv6 subnet?
Because SLAAC and many IPv6 features assume 64 host bits (a 64-bit interface identifier). Using anything longer than /64 for a LAN can break address auto-configuration.
How many /64 subnets are in a /48?
2 to the power (64 − 48) = 2^16 = 65,536 subnets.
Do you subtract 2 for hosts in IPv6?
No. IPv6 has no broadcast address, so the network/broadcast subtraction from IPv4 does not apply. A /64 holds an astronomically large number of addresses.
What is a nibble boundary?
A prefix length that is a multiple of 4 (/52, /56, /60, /64). Because each hex digit is 4 bits, subnetting on these boundaries changes exactly one hex digit and keeps the maths simple.
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