Wildcard mask

A wildcard mask (or a host mask) Identifies the host bits in an IP address (the opposite of a subnet mask)
- HOWEVER, Wildcard mask =/= subnet mask
  - A subnet has a network address, broadcast address and follows other kinds of rules
  - A wildcard mask is literally "Any host with an IP address in this range"
    - e.g., in OSPF, the network command looks for any interfaces IP that falls in the wildcard mask range, and then advertises the network associated with that interface.
      - It does NOT advertise the inverse-network of the wildcard mask
    - Example: config-router# network 10.0.0.0 0.255.255.255 area 0
      - It DOES grab any IP address that falls between 10.0.0.0 and 10.255.255.255, adds them to Area 0, and advertises their networks
      - It does NOT advertise 10.0.0.0/8 on area 0
Two ways to calculate the wildcard mask
- Full: Flip the subnet bits
  - 11111111.11111111.11111000.00000000
    - 255.255.248.0
  - 00000000.00000000.00000111.11111111
    - 0.0.7.255
- Shortcut: Subtract 255 from each octet (and make positive)
  - 255.255.192.0
  - 0.0.63.255
Wildcard masks are often used in dynamic routing protocols (like EIGRP and OSPF) and in ACLs

Metadata