The /17 prefix means the first 17 bits of the address identify the network and the remaining 15 identify the host. Written out, the mask is 255.255.128.0, in binary 11111111.11111111.10000000.00000000, and its Cisco wildcard or inverse form is 0.0.127.255. It covers 32,768 addresses, of which 32,766 can be given to hosts.
| CIDR prefix | /17 |
|---|---|
| Subnet mask | 255.255.128.0 |
| Mask in binary | 11111111.11111111.10000000.00000000 |
| Wildcard (inverse) mask | 0.0.127.255 |
| Host bits | 15 |
| Total addresses | 32,768 |
| Usable hosts | 32,766 |
| Block size (magic number) | 128 in octet 3 |
A 172.16.0.0/16 splits into exactly 2 subnets of this size.
| Network | First host | Last host | Broadcast |
|---|---|---|---|
| 172.16.0.0/17 | 172.16.0.1 | 172.16.127.254 | 172.16.127.255 |
| 172.16.128.0/17 | 172.16.128.1 | 172.16.255.254 | 172.16.255.255 |
Take the interesting octet of the mask, the one that is neither 255 nor 0. For /17 that is 128 in octet 3. Subtract it from 256:
256 - 128 = 128
That is the block size. Every /17 network address is a multiple of 128 in octet 3, so you can list them by counting: 0, 128, 256, 384, and so on. No binary needed once you know the number.
A /17 is half a site block. It appears when a /16 is divided cleanly in two, typically to separate infrastructure from user space, or to hand one half to a second data centre while keeping summarisation simple.
A /17 holds 32,768 addresses in total. After reserving the network and broadcast addresses, 32,766 are usable by hosts.
The /17 subnet mask is 255.255.128.0 in dotted-decimal notation.
The wildcard mask, also called the inverse mask, is 0.0.127.255. It is the bitwise inverse of 255.255.128.0 and is what Cisco access lists and OSPF network statements expect.
The /17 mask written out bit by bit is 11111111.11111111.10000000.00000000: 17 ones followed by 15 zeros.