Is this IP address in the subnet?
Enter an IPv4 address and a network in CIDR form, for example 10.0.0.0/8, and see whether the address belongs to it. It runs in your browser; nothing is sent anywhere.
Yes: 10.20.30.40 is inside 10.0.0.0/8.
Addresses in that network: 10.0.0.0 – 10.255.255.255 (16,777,216)
It is address number 1,318,441 of 16,777,216.
Questions
How does the check work?
The address and the network are compared after keeping only the network bits of each. If they are the same, the address is inside.
What if the network has host bits set?
They are ignored. 192.168.1.77/24 is treated as 192.168.1.0/24, the network that address belongs to.
Does it work for IPv6?
Not this tool. For IPv6 prefixes use the IPv6 subnet calculator.