IPv6
/81
2^47
IPv6 /81 prefix
/81 means the first 81 bits are the network and 47 are left for hosts. That is 2^47 addresses.
Addresses are 128 bits, so nobody writes a mask — only the prefix length. A home gets a /56, a single network a /64.
- Prefix
- /81
- Host bits
- 47
- Addresses
- 2^47
- Usable hosts
- 2^47
- /64 networks inside
- None
- Nibble boundary
- No
Prefixes either side
How to read this
- /24 means the first 24 bits are the network; the remaining 8 hold the addresses inside it.
- Each extra bit splits the network in two and halves the addresses.
- Usable addresses are normally the total minus two — the network and broadcast addresses.
- IPv6 has no broadcast, so nothing is subtracted. One network is conventionally a /64.
Frequently asked questions
Q. How many addresses are in a /81?
2^47, of which 2^47 can actually be handed out.
Q. How is a /81 written?
IPv6 uses no mask — you simply write /81.
Q. How many /64 networks fit in a /81?
None — it is narrower than a /64.
Q. How many host bits are there?
47. Each extra network bit halves the addresses.