Quiz Entry - updated: 2026.07.14
What is the IPv6 prefix length and what is the recommended prefix for LANs?
The prefix length (slash notation, 0-128) marks the network portion — there is no dotted-decimal mask in IPv6. The recommended LAN prefix is /64.
IPv6 Prefix Length:
Similar to IPv4 subnet mask - indicates the network portion.
Format: IPv6-address**/prefix-length**
| Notation | Network Bits | Interface ID Bits |
|---|---|---|
| /48 | 48 | 80 |
| /64 | 64 | 64 |
| /128 | 128 | 0 (single host) |
Recommended prefix: /64
| Part | Bits | Purpose |
|---|---|---|
| Network prefix | 64 | Identifies the network/subnet |
| Interface ID | 64 | Identifies the specific host |
Why /64 is standard:
- Required for SLAAC (auto-configuration) to work
- Provides 2^64 addresses per subnet (18 quintillion)
- More than enough for any network
Example:
2001:db8:acad:1::1/64
├─────────────────┤├┤
Network (64) Host (64)
Go deeper:
RFC 4291 §2.4 — Address Type Identification — the prefix-length convention that marks the network portion (no dotted mask in IPv6).- IPv6 address — subnets (Wikipedia) — why /64 is the standard LAN size and how it splits into 64-bit network and interface halves.