IPV4 ADDRESSING

IP address are of two types-
Public and private.
PUBLIC IP ADDRESS- They are meant to routable (used for internet), they are registered and paid too. 
PRIVATE IP ADDRESS- They are meant to non-routable, unregistered and available free of cost.
IP addresses are divided into five classes—

A to E
Private IP addresses are declared-
Range of Private IP addresses—

1. A-  10.0.0.0 to 10.255.255.255
   
2. B-  172.16.0.0 to 172.31.255.255
   
3. C-  192.168.0.0 to 192.168.255.255
   
4. D-  Used as multicast address only
   
5. E-  Reserved for research and documentation. 
   
   
   The range of first octet (public +private)—
   A- 1-126 
   B- 128-191 
   C- 192-223 
   D- 224-239 
   E- 240-255
   Note- Usable IP address classes are A to C. DEFAULT SUBNET MASKS
   A- 255.0.0.0
   B- 255.255.0.0 
   C- 255.255.255.0
   Subnet masks are assigned to with IP address of its classes, they are very important.