AWS Certified Solutions Architect Associate - Exam TIPS
3 Types of Load Balancers
Application Load Balancers Network Load Balancers Classic Load Balancers 504 Error means the gateway has timed out. Application is not responding within the idle timeout period
Trouble shoot the application. Web Server or Database Server? If you need IPv4 address of your end user, look fro the X-Forwarded-For header.
Instances are monitored but ELB are reported as InService or OutofService.
Health Checks check the instance health by talking to it.
ELB's have their own DNS name. You are never given an IP address
Read the ELB FAQ for Classic Load Balancers
Note: ELB's do not have IP Addresses, only found by DNS namespace