IPv4
/17
255.255.128.0
/17 subnet — mask 255.255.128.0
/17 means the first 17 bits are the network and 15 are left for hosts. That is 32,768 addresses, with mask 255.255.128.0.
Addresses are 32 bits, so the mask is written in four dotted parts: 255.255.255.0.
- Prefix
- /17
- Subnet mask
- 255.255.128.0
- Wildcard
- 0.0.127.255
- Mask in hex
- FFFF8000
- Host bits
- 15
- Addresses
- 32,768
- Usable hosts
- 32,766
- /24 networks inside
- 128
- Nibble boundary
- No
Seen as bits
The leading ones are the network part and the trailing zeros the host part. Add one more one and the network splits in two, halving the addresses.
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 /17?
32,768, of which 32,766 can actually be handed out.
Q. What is the subnet mask for /17?
255.255.128.0, with wildcard 0.0.127.255.
Q. How many /24 networks fit in a /17?
128.
Q. How many host bits are there?
15. Each extra network bit halves the addresses.