Network Subnetting

Calculating a Subnet Mask from an IP Address with CIDR Notation

1. Convert the CIDR notation to binary
   1. A /24 CIDR becomes 11111111.11111111.11111111.00000000
   2. A /25 CIDR becomes 11111111.11111111.11111111.10000000
   3. a /30 CIDR becomes 11111111.11111111.11111111.11111100
2. Once in binary, calculate the total binary values of each octet:
   1. This can be achieved by adding up the decimal values for each of the bits based on the ruler outlined below.
      1. A /24 CIDR with a binary value of 11111111.11111111.11111111.00000000 has a subnet mask of 255.255.255.0
      2. A /25 CIDR with a binary value of 11111111.11111111.11111111.10000000 has a subnet mask of 255.255.255.128
      3. A /30 CIDR with a binary value of 11111111.11111111.11111111.11111100 has a subnet mask of 255.255.255.252

         Subnet Ruler

         Each binary octet (8 bits) below will use the following values from left to right to determine the total decimal value of the octet: 128-64-32-16-8-4-2-1

Calculating the Wildcard Mask using the Subnet Mask

1. For each octet in the subnet mask, subtract the decimal value from 255.
   1. The wildcard mask for the /24 subnet mask (255.255.255.0) is 0.0.0.255
   2. The wildcard mask for the /30 subnet mask (255.255.255.252) is 0.0.0.3