Practice
Work backwards
Start from hosts, subnets or a mask and find the prefix. The skill you need when sizing real networks.
Four problems that start from a requirement or a mask and ask for the prefix.
Prefix vs mask vs wildcard
The prefix (/26) counts the network bits. The mask writes those bits as four numbers (255.255.255.192). The wildcard is the mask flipped (0.0.0.63), used by ACLs and OSPF. Host bits are whatever is left: 32 − prefix.