When we subnet a class A, B, or C address, we identify the change using custom subnet mask. Although we use the term custom subnet mask, in your career, you’ll just identify it as the subnet mask. A custom subnet mask is used by equipment and IT professionals to determine what the network range is for any given network.
In this exercise, we’ll determine the CIDR notation and custom subnet mask for various addresses and the number of bits being borrowed. You can use the following process to determine the answers:
- Identify if the address is a class A, B, or C
- Determine if there are 8, 16, or 24 bits network bits for the default mask
- Add the default network bits to the ‘Borrow’ bits listed in the table to determine the CIDR notation
- Determine the ‘Custom Subnet Mask’ based off total number of network bits. You can use the borrowed bits to decimal table on the left as a reference to determine the ‘Custom Subnet Mask’
Check your answers with the answer key below. It’s recommended that before you proceed, you could perform this exercise without the use of the table. If you need additional practice, use the worksheet found at TechknowSurge.com. (link below).
Worksheet: https://techknowsurge.com/resources/classes/ip-subnetting/Custom-Subnet-Masks-Worksheet.pdf
Worksheet: https://techknowsurge.com/resources/classes/ip-subnetting/Custom-Subnet-Masks-Worksheet.pdf