How to use this on a job
Two addresses that look similar can still be on different subnets. The prefix decides which bits describe the network. Compare the masked addresses before changing cables or inventing a gateway.
- 01
Enter the interface’s IPv4 address and CIDR prefix. Use plain decimal octets without leading zeros so there is no ambiguity with other numeric notations.
- 02
Read the network, broadcast and usable range. For ordinary subnets, the network and broadcast addresses are not assignable host addresses.
- 03
Enter a peer address to test membership in this interface’s subnet. Membership is only one side of the configuration: the peer’s mask, VLAN, firewall and physical link still matter.
Work through one example
/27 → 255.255.255.224 → blocks of 32 addresses
192.168.10.35/27 belongs to network 192.168.10.32. Its broadcast is .63 and its ordinary hosts run from .33 through .62. A peer at .63 matches the network bits but is not a usable host. A peer at .64 is already in the next /27 subnet.
Before you trust the answer
Being in the same subnet does not prove reachability. An address at the broadcast boundary belongs numerically to the subnet but cannot be assigned as an ordinary host. The calculator reports these as separate facts. Use /31 only with devices and links that support that point-to-point convention.