Subnet masks and CIDR notation

Subnet masks and CIDR notation describe how a host identifies its local network, neighbours and route to other networks. IPv4 depends on the address, prefix length and gateway; IPv6 also depends on address scope and Neighbor Discovery. One wrong value can leave local communication working while breaking access beyond the subnet.

How to check the network settings

Start with the interface address, prefix, routing table and default gateway. Check whether the destination belongs to the local prefix and whether a default route exists. Do not change a mask or gateway by guesswork; compare it with DHCP or network documentation and repeat the same test after one controlled change.

The detail that matters

For 192.168.1.0/24 the first 24 bits identify the network and the remaining eight bits identify addresses inside it. Moving to /25 creates two 128-address blocks; /26 creates four 64-address blocks. IPv6 commonly uses /64 on LANs, but IPv6 has no IPv4-style broadcast address. For a calculation you can compare with the explanation above, use 2ip IP calculator.