The /19 prefix means the first 19 bits of the address identify the network and the remaining 13 identify the host. Written out, the mask is 255.255.224.0, in binary 11111111.11111111.11100000.00000000, and its Cisco wildcard or inverse form is 0.0.31.255. It covers 8,192 addresses, of which 8,190 can be given to hosts.
| CIDR prefix | /19 |
|---|---|
| Subnet mask | 255.255.224.0 |
| Mask in binary | 11111111.11111111.11100000.00000000 |
| Wildcard (inverse) mask | 0.0.31.255 |
| Host bits | 13 |
| Total addresses | 8,192 |
| Usable hosts | 8,190 |
| Block size (magic number) | 32 in octet 3 |
A 172.16.0.0/16 splits into exactly 8 subnets of this size.
| Network | First host | Last host | Broadcast |
|---|---|---|---|
| 172.16.0.0/19 | 172.16.0.1 | 172.16.31.254 | 172.16.31.255 |
| 172.16.32.0/19 | 172.16.32.1 | 172.16.63.254 | 172.16.63.255 |
| 172.16.64.0/19 | 172.16.64.1 | 172.16.95.254 | 172.16.95.255 |
| 172.16.96.0/19 | 172.16.96.1 | 172.16.127.254 | 172.16.127.255 |
| 172.16.128.0/19 | 172.16.128.1 | 172.16.159.254 | 172.16.159.255 |
| 172.16.160.0/19 | 172.16.160.1 | 172.16.191.254 | 172.16.191.255 |
| 172.16.192.0/19 | 172.16.192.1 | 172.16.223.254 | 172.16.223.255 |
| 172.16.224.0/19 | 172.16.224.1 | 172.16.255.254 | 172.16.255.255 |
Take the interesting octet of the mask, the one that is neither 255 nor 0. For /19 that is 224 in octet 3. Subtract it from 256:
256 - 224 = 32
That is the block size. Every /19 network address is a multiple of 32 in octet 3, so you can list them by counting: 0, 32, 64, 96, and so on. No binary needed once you know the number.
A /19 was for years the smallest IPv4 block a registry would allocate, which is why so many older provider allocations are exactly this size. 8,190 usable addresses.
A /19 holds 8,192 addresses in total. After reserving the network and broadcast addresses, 8,190 are usable by hosts.
The /19 subnet mask is 255.255.224.0 in dotted-decimal notation.
The wildcard mask, also called the inverse mask, is 0.0.31.255. It is the bitwise inverse of 255.255.224.0 and is what Cisco access lists and OSPF network statements expect.
The /19 mask written out bit by bit is 11111111.11111111.11100000.00000000: 19 ones followed by 13 zeros.