EKS: verify connection between ec2 and rds instances and difference between nslookup and telnet

I have setup a RDS instance and I am trying to see if i am able to connect from my EC2 instances(setup using EKS) to the RDS instances.

All my EC2 and RDS instances are in the same VPC.

There is a security group associated with my RDS to which I have set the following inbound and outbound rules.

Inbound: PostgreSQL 5432 192.168.0.0/32

Outbound: All TCP 0-65535 192.168.0.0/32

I am trying to check if I am able to connect from my ec2 instances to the RDS

when i execute

kubectl exec -it debug -- telnet xyz.us-east-1.rds.amazonaws.com 5432 the output is Connected to xyz.us-east-1.rds.amazonaws.com

but when I execute kubectl exec -it debug -- nslookup xyz.us-east-1.rds.amazonaws.com:5432 the output is can't find xyz.us-east-1.rds.amazonaws.com

1. Why does nslookup not work and telnet work?

2. Is telnet a correct way to verify that my EC2 instances will be able to talk to my RDS instances?

Any help on clarifying this would be really great. Thanks.