Subnets·IPv6 /100
IPv6
/100
268,435,456

IPv6 /100 prefix

/100 means the first 100 bits are the network and 28 are left for hosts. That is 268,435,456 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
/100
Host bits
28
Addresses
268,435,456
Usable hosts
268,435,456
/64 networks inside
None
Nibble boundary
Yes

Prefixes either side

/972,147,483,6482,147,483,648/981,073,741,8241,073,741,824/99536,870,912536,870,912/101134,217,728134,217,728/10267,108,86467,108,864/10333,554,43233,554,432

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 /100?

268,435,456, of which 268,435,456 can actually be handed out.

Q. How is a /100 written?

IPv6 uses no mask — you simply write /100.

Q. How many /64 networks fit in a /100?

None — it is narrower than a /64.

Q. How many host bits are there?

28. Each extra network bit halves the addresses.