Subnets
A subnet is a segment of an IP address space.
A subnet looks like the following:
| CIDR notation | 10.20.0.0/24 |
| Subnet mask | 255.255.255.0 |
Subnet Mask
The mask in this sense is like multiplying by 1 (aka this bit exists)
| 00001010.00010100.00000000.00000000 |
| 11111111.11111111.11111111.00000000 x |
| =00001010.00010100.00000000.00000000 |
So, in essence the first 3 bytes from the top IP address will always be the same, however, the subnet mask has no effect on the last byte.
From the CIDR notation, note the /24 at the end, this signifies 24 bits for the network prefix of the IP address, leaving 8 bits for the host identifier (32bits - 24bits).
In simpler terms, you could give a machine an ip address anywhere between 10.20.0.0 and 10.20.0.255.