1)Airtel provides both static & dynamic addresses, Dynamic IP is default

2)If u want to assign static then use the following IP range mentioned below

3) 192.168.1.2 till 192.168.1.254

4)Do not use 192.1668.1.1 IP Bcoz it is assigned to default gateway

5)Assign DNS address also for the internet to work properly

6)For DNS address ask airtel customer care it's provided free