IPv6
/40
2^88
IPv6 /40 prefix
/40 means the first 40 bits are the network and 88 are left for hosts. That is 2^88 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
- /40
- Host bits
- 88
- Addresses
- 2^88
- Usable hosts
- 2^88
- /64 networks inside
- 2^88
- Nibble boundary
- Yes
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 /40?
2^88, of which 2^88 can actually be handed out.
Q. How is a /40 written?
IPv6 uses no mask — you simply write /40.
Q. How many /64 networks fit in a /40?
2^88.
Q. How many host bits are there?
88. Each extra network bit halves the addresses.