Subnetting Best Practice From Largest to Smallest

When subnetting a larger IPv4 address space into smaller subnets, it is usually best practice to begin subnetting from the largest subnet to the smallest. This is done to prevent potential issues in address assignment.

When allocating subnets, starting with the largest required network ensures that there will be enough space for all necessary hosts and reduces the risk of generating invalid network addresses.

If smaller subnets are assigned first, the remaining address space may become fragmented, making it impossible to allocate contiguous blocks for larger subnets. This can lead to overlaps or invalid network boundaries.

For example, if you first allocate a /28 subnet (e.g., 192.168.1.0/28), the next available block starts at 192.168.1.16. If you later try to assign a /25 subnet (which requires a block of 128 addresses starting at 192.168.1.0), it would conflict with the existing /28 allocation, creating an invalid overlap.

By prioritizing larger subnets first, this issue is avoided.

Links

https://networklessons.com/subnetting/variable-length-subnet-mask-vlsm

https://networklessons.com/subnetting/what-is-subnetting

https://networklessons.com/subnetting/basics-of-binary-numbers

https://networklessons.com/subnetting/subnetting-in-binary

https://networklessons.com/subnetting/subnetting-in-decimal-fast-way