The /18 prefix means the first 18 bits of the address identify the network and the remaining 14 identify the host. Written out, the mask is 255.255.192.0, in binary 11111111.11111111.11000000.00000000, and its Cisco wildcard or inverse form is 0.0.63.255. It covers 16,384 addresses, of which 16,382 can be given to hosts.
| CIDR prefix | /18 |
|---|---|
| Subnet mask | 255.255.192.0 |
| Mask in binary | 11111111.11111111.11000000.00000000 |
| Wildcard (inverse) mask | 0.0.63.255 |
| Host bits | 14 |
| Total addresses | 16,384 |
| Usable hosts | 16,382 |
| Block size (magic number) | 64 in octet 3 |
A 172.16.0.0/16 splits into exactly 4 subnets of this size.
| Network | First host | Last host | Broadcast |
|---|---|---|---|
| 172.16.0.0/18 | 172.16.0.1 | 172.16.63.254 | 172.16.63.255 |
| 172.16.64.0/18 | 172.16.64.1 | 172.16.127.254 | 172.16.127.255 |
| 172.16.128.0/18 | 172.16.128.1 | 172.16.191.254 | 172.16.191.255 |
| 172.16.192.0/18 | 172.16.192.1 | 172.16.255.254 | 172.16.255.255 |
Take the interesting octet of the mask, the one that is neither 255 nor 0. For /18 that is 192 in octet 3. Subtract it from 256:
256 - 192 = 64
That is the block size. Every /18 network address is a multiple of 64 in octet 3, so you can list them by counting: 0, 64, 128, 192, and so on. No binary needed once you know the number.
A /18 holds 16,382 usable addresses, a quarter of a /16. It is a reasonable size for a whole building or a large cloud subnet, and a common size for the block an ISP assigns to a substantial business customer.
A /18 holds 16,384 addresses in total. After reserving the network and broadcast addresses, 16,382 are usable by hosts.
The /18 subnet mask is 255.255.192.0 in dotted-decimal notation.
The wildcard mask, also called the inverse mask, is 0.0.63.255. It is the bitwise inverse of 255.255.192.0 and is what Cisco access lists and OSPF network statements expect.
The /18 mask written out bit by bit is 11111111.11111111.11000000.00000000: 18 ones followed by 14 zeros.