Are two IPv4 addresses on the same subnet? The first three octets are not enough
Subnet membership depends on the prefix. Matching the first three octets is sufficient only under particular masks, not as a universal rule.
IPv4 subnet calculator →Compare two addresses with a /26 prefix
192.168.1.10/26 belongs to the block from 192.168.1.0 through 192.168.1.63. The address 192.168.1.70/26 belongs to the next block, from 192.168.1.64 through 192.168.1.127. Their first three octets match, yet they are in different /26 networks. The last octet contains some network bits under that prefix.
Changing the prefix changes the question
With /24 applied to both addresses, their network address becomes 192.168.1.0. Use the actual configured prefix consistently when comparing devices; do not alter a mask merely to obtain a matching result. Enter each address separately and compare the calculator’s network-address output. Keep the prefix with any copied address so the comparison remains reproducible.
Common membership does not guarantee connectivity
Firewalls, VLANs, routing, device state and duplicate addresses can still affect communication. This calculator does not scan devices or test a live network. It checks the arithmetic relationship between an IPv4 address and the supplied prefix. A bandwidth estimate answers a different capacity question and also cannot diagnose reachability. Use appropriate network diagnostics and configuration information when communication fails despite a matching subnet calculation.
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