AWS Elastic IP

for allocating static IP on EC2 or some instance

you need to use Elastic IP

An Elastic IP address is a static public IPv4 address associated with your AWS account in a specific Region. Unlike an auto-assigned public IP address, an Elastic IP address is preserved after you stop and start your instance in a virtual private cloud (VPC).

https://aws.amazon.com/premiumsupport/knowledge-center/ec2-associate-static-public-ip/