To work with your AWS RDS PostgreSQL instance, you will need to provide your IP address or range. If you’ve got a dynamic IP address, this is the process to follow:
1. Determine your current IP: http://www.whatsmyip.org/
2. Look up the IP address range assigned to your ISP: https://apps.db.ripe.net/search/query.html
3. Convert that range into CIDR notation: http://www.ipaddressguide.com/cidr
4. Enter that range (or ranges) into your EC2 Security Group.
With thanks to Andy