Quiz Entry - updated: 2026.07.14
How is a typical 128-bit IPv6 address split between network and host?
For end systems it's a clean 50/50 split: the first 64 bits are the network/subnet prefix, the last 64 bits are the interface identifier.
| Part | Bits | Identifies |
|---|---|---|
| Subnet prefix | first /64 | the broadcast domain / link (e.g. one residence, one LAN) |
| Interface ID | last 64 | the specific machine on that link |
Because the host part is a full 64 bits, each subnet can hold ~18 quintillion (1.8 × 10¹⁹) hosts — you essentially never run out of host addresses within a subnet, which is why IPv6 subnetting is about organisation, not conservation.
Tip: "/64 is the magic number" — SLAAC and many IPv6 features assume a 64-bit interface ID, so a standard LAN is almost always a /64.
Go deeper:
RFC 4291 §2.5.1 — Interface Identifiers — why the interface ID is 64 bits, fixing the /64 boundary.