The /15 prefix means the first 15 bits of the address identify the network and the remaining 17 identify the host. Written out, the mask is 255.254.0.0, in binary 11111111.11111110.00000000.00000000, and its Cisco wildcard or inverse form is 0.1.255.255. It covers 131,072 addresses, of which 131,070 can be given to hosts.
| CIDR prefix | /15 |
|---|---|
| Subnet mask | 255.254.0.0 |
| Mask in binary | 11111111.11111110.00000000.00000000 |
| Wildcard (inverse) mask | 0.1.255.255 |
| Host bits | 17 |
| Total addresses | 131,072 |
| Usable hosts | 131,070 |
| Block size (magic number) | 2 in octet 2 |
A 10.0.0.0/8 contains 128 subnets of /15. Here are the first 8, which is enough to see the pattern: each one starts 2 after the last in octet 2.
| Network | First host | Last host | Broadcast |
|---|---|---|---|
| 10.0.0.0/15 | 10.0.0.1 | 10.1.255.254 | 10.1.255.255 |
| 10.2.0.0/15 | 10.2.0.1 | 10.3.255.254 | 10.3.255.255 |
| 10.4.0.0/15 | 10.4.0.1 | 10.5.255.254 | 10.5.255.255 |
| 10.6.0.0/15 | 10.6.0.1 | 10.7.255.254 | 10.7.255.255 |
| 10.8.0.0/15 | 10.8.0.1 | 10.9.255.254 | 10.9.255.255 |
| 10.10.0.0/15 | 10.10.0.1 | 10.11.255.254 | 10.11.255.255 |
| 10.12.0.0/15 | 10.12.0.1 | 10.13.255.254 | 10.13.255.255 |
| 10.14.0.0/15 | 10.14.0.1 | 10.15.255.254 | 10.15.255.255 |
Take the interesting octet of the mask, the one that is neither 255 nor 0. For /15 that is 254 in octet 2. Subtract it from 256:
256 - 254 = 2
That is the block size. Every /15 network address is a multiple of 2 in octet 2, so you can list them by counting: 0, 2, 4, 6, and so on. No binary needed once you know the number.
A /15 is two adjacent /16s treated as one block. Companies end up with one when they outgrow a /16 and take the neighbouring block so the pair can be summarised into a single route.
A /15 holds 131,072 addresses in total. After reserving the network and broadcast addresses, 131,070 are usable by hosts.
The /15 subnet mask is 255.254.0.0 in dotted-decimal notation.
The wildcard mask, also called the inverse mask, is 0.1.255.255. It is the bitwise inverse of 255.254.0.0 and is what Cisco access lists and OSPF network statements expect.
The /15 mask written out bit by bit is 11111111.11111110.00000000.00000000: 15 ones followed by 17 zeros.