Run each IP with the shared CIDR prefix through a browser-based subnet calculator and compare the network address it returns; if both give the same network address, they are in the same subnet. It runs on your device with nothing uploaded.
Two IP addresses belong to the same subnet when, under the same prefix, they share the same network address. The prefix decides how many leading bits count as the network portion, and the subnet calculator applies that mask to each IP to reveal its network address. If the two network addresses match, the hosts can reach each other directly without a router; if they differ, traffic has to be routed.
A client-side subnet calculator does the masking in your browser, so you can compare private addresses without sending them to a server. Enter the first IP with the prefix, note its network address, then do the same for the second IP and compare. It is free and needs no account.
Apply the same prefix to both and compare their network addresses; matching means the same subnet.
Yes. The same two IPs can be in one subnet under one prefix and different subnets under another.
No. The calculation runs in your browser, so nothing is sent to a server.