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Wildcard Mask

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.

 


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