The /23 prefix means the first 23 bits of the address identify the network and the remaining 9 identify the host. Written out, the mask is 255.255.254.0, in binary 11111111.11111111.11111110.00000000, and its Cisco wildcard or inverse form is 0.0.1.255. It covers 512 addresses, of which 510 can be given to hosts.
| CIDR prefix | /23 |
|---|---|
| Subnet mask | 255.255.254.0 |
| Mask in binary | 11111111.11111111.11111110.00000000 |
| Wildcard (inverse) mask | 0.0.1.255 |
| Host bits | 9 |
| Total addresses | 512 |
| Usable hosts | 510 |
| Block size (magic number) | 2 in octet 3 |
A 172.16.0.0/16 contains 128 subnets of /23. Here are the first 8, which is enough to see the pattern: each one starts 2 after the last in octet 3.
| Network | First host | Last host | Broadcast |
|---|---|---|---|
| 172.16.0.0/23 | 172.16.0.1 | 172.16.1.254 | 172.16.1.255 |
| 172.16.2.0/23 | 172.16.2.1 | 172.16.3.254 | 172.16.3.255 |
| 172.16.4.0/23 | 172.16.4.1 | 172.16.5.254 | 172.16.5.255 |
| 172.16.6.0/23 | 172.16.6.1 | 172.16.7.254 | 172.16.7.255 |
| 172.16.8.0/23 | 172.16.8.1 | 172.16.9.254 | 172.16.9.255 |
| 172.16.10.0/23 | 172.16.10.1 | 172.16.11.254 | 172.16.11.255 |
| 172.16.12.0/23 | 172.16.12.1 | 172.16.13.254 | 172.16.13.255 |
| 172.16.14.0/23 | 172.16.14.1 | 172.16.15.254 | 172.16.15.255 |
Take the interesting octet of the mask, the one that is neither 255 nor 0. For /23 that is 254 in octet 3. Subtract it from 256:
256 - 254 = 2
That is the block size. Every /23 network address is a multiple of 2 in octet 3, so you can list them by counting: 0, 2, 4, 6, and so on. No binary needed once you know the number.
A /23 is the double /24: 510 usable addresses across two consecutive class C blocks. It exists almost entirely because a user VLAN filled up its /24 and extending the mask by one bit was cheaper than splitting the VLAN.
A /23 holds 512 addresses in total. After reserving the network and broadcast addresses, 510 are usable by hosts.
The /23 subnet mask is 255.255.254.0 in dotted-decimal notation.
The wildcard mask, also called the inverse mask, is 0.0.1.255. It is the bitwise inverse of 255.255.254.0 and is what Cisco access lists and OSPF network statements expect.
The /23 mask written out bit by bit is 11111111.11111111.11111110.00000000: 23 ones followed by 9 zeros.