There are five classes of IP: class A, B, C, D, E. Class D, and E are not mentioned here, but only A, B, and C are the most commonly used in the world.
1 to 126 is class A (referring to First Octet)
B class from 128 to 191 (referring to First Octet)
C class 192 to 233 (referring to First Octet)
Private IP and Public
First Octet starts with 10 and is a private IP
First Octet starts with 172 and is a private IP
First Octet is a private IP starting with 192
First Octet starts with 169 and is a private IP
First Octet starts with 127 and is a private IP (localhost).
If you know these private IPs, you will be using the Private IP group you want to use depending on your LAN network.