Using a subnet calculator and checking whether two hosts are on the same subnet

Whether two devices are on the same subnet decides how they talk. If they share a subnet, the sender delivers frames directly using ARP to find the other device's MAC address. If they are on different subnets, the sender must hand the packet to its default gateway. A mask typo that puts a host in the wrong subnet causes confusing, one-sided failures, so checking this quickly is a practical help desk skill.