VLSM subnetting

 

https://www.tutorialspoint.com/ipv4/ipv4_vlsm.htm

https://www.geeksforgeeks.org/introduction-of-variable-length-subnet-mask-vlsm/

https://www.computernetworkingnotes.com/ccna-study-guide/vlsm-subnetting-explained-with-examples.html