Overview:
A wildcard mask is a mask of bits (a bitwise operation) that indicates which parts of an IP address are available for examination.
Study Notes:
- A bitwise operation is one that compares two bits with each other to come up with a result - IP and mask (subnet mask, wildcard mask, etc)
- Mask - a string of bits that are used to apply a bitwise operation to another string of bits
- Subnet mask - a bitwise AND operation applied to an IP address
- Wildcard mask - a bitwise XOR operation applied to an IP address
- Bitwise NOT - flips the bit (logical negation)
| IP | RESULT | |
| 0 | 1 | |
| 1 | 0 |
- Bitwise AND - multiplies the bits (logical AND)
| IP | MASK | RESULT |
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
- Bitwise OR - if any are 1 the result is 1 (logical inclusive OR)
| IP | MASK | RESULT |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
- Bitwise XOR - if the values match the result is 0 (logical exclusive OR)
| IP | MASK | RESULT |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
Examples:
Can be used in OSPF commands and access-list commands
| IP | WILDCARD MASK | RESULT |
| 192.168.0.0/32 | 0.0.0.0 | 192.168.0.1 specifically |
| 192.168.0.0/24 | 0.0.0.255 | 192.168.0.0 network |
| 172.16.0.0/24 to 172.16.15.0/24 | 0.0.15.255 | 172.16.0.1 to 172.16.15.255 |
| 172.16.16.0 t0 172.16.31.0 | 0.0.15.255 | 172.16.16.1 to 172.16.31.255 |
| 172.16.63.0 to 172.16.66.0 | 172.16.63.0 0.0.0.255
172.16.64.0 0.0.1.255 172.16.66.0 0.0.0.255 |
Three wildcard masks are required to explicity address these four networks because we run into a newtork boundary at 172.16.64.0. |