The /11 prefix means the first 11 bits of the address identify the network and the remaining 21 identify the host. Written out, the mask is 255.224.0.0, in binary 11111111.11100000.00000000.00000000, and its Cisco wildcard or inverse form is 0.31.255.255. It covers 2,097,152 addresses, of which 2,097,150 can be given to hosts.
| CIDR prefix | /11 |
|---|---|
| Subnet mask | 255.224.0.0 |
| Mask in binary | 11111111.11100000.00000000.00000000 |
| Wildcard (inverse) mask | 0.31.255.255 |
| Host bits | 21 |
| Total addresses | 2,097,152 |
| Usable hosts | 2,097,150 |
| Block size (magic number) | 32 in octet 2 |
A 10.0.0.0/8 splits into exactly 8 subnets of this size.
| Network | First host | Last host | Broadcast |
|---|---|---|---|
| 10.0.0.0/11 | 10.0.0.1 | 10.31.255.254 | 10.31.255.255 |
| 10.32.0.0/11 | 10.32.0.1 | 10.63.255.254 | 10.63.255.255 |
| 10.64.0.0/11 | 10.64.0.1 | 10.95.255.254 | 10.95.255.255 |
| 10.96.0.0/11 | 10.96.0.1 | 10.127.255.254 | 10.127.255.255 |
| 10.128.0.0/11 | 10.128.0.1 | 10.159.255.254 | 10.159.255.255 |
| 10.160.0.0/11 | 10.160.0.1 | 10.191.255.254 | 10.191.255.255 |
| 10.192.0.0/11 | 10.192.0.1 | 10.223.255.254 | 10.223.255.255 |
| 10.224.0.0/11 | 10.224.0.1 | 10.255.255.254 | 10.255.255.255 |
Take the interesting octet of the mask, the one that is neither 255 nor 0. For /11 that is 224 in octet 2. Subtract it from 256:
256 - 224 = 32
That is the block size. Every /11 network address is a multiple of 32 in octet 2, so you can list them by counting: 0, 32, 64, 96, and so on. No binary needed once you know the number.
A /11 carries two million addresses. In practice it appears as a regional slice of a corporate 10.0.0.0/8, for example a continent or a business unit, cut once more before anything is actually assigned to a subnet.
A /11 holds 2,097,152 addresses in total. After reserving the network and broadcast addresses, 2,097,150 are usable by hosts.
The /11 subnet mask is 255.224.0.0 in dotted-decimal notation.
The wildcard mask, also called the inverse mask, is 0.31.255.255. It is the bitwise inverse of 255.224.0.0 and is what Cisco access lists and OSPF network statements expect.
The /11 mask written out bit by bit is 11111111.11100000.00000000.00000000: 11 ones followed by 21 zeros.