Type any CIDR block, like 10.0.0.0/16, and instantly see the full address range it covers, how many hosts fit inside it, and the subnet mask in both normal and hex form.
The prefix number tells us how many of the 32 bits are fixed (the network part).
The remaining bits are the host part, and they decide how big the block is.
Setting all host bits to 0 gives the first address. Setting them all to 1 gives the last address.
First address: Network address
Last address: Network + 2 ^ (32 − prefix) − 1
Block size: 2 ^ (32 − prefix)
What does the number after the slash mean?
It tells you how many bits are fixed as the network part. A /16 fixes the first 16 bits, leaving 16 bits for hosts — that's 65,536 addresses in total.
Which private ranges can I use on an internal network?
You can use 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. These are set aside for private use by RFC 1918 and cannot be used on the public internet.