Can Instances in the same security groups can communicate each other any how

Question

Can Instances in the same security groups in Amazon VPC can communicate each other any how

Answer 1

Rules to connect to instances from an instance with the same security group

To allow instances that are associated with the same security group to communicate with each other, you must explicitly add rules for this.

The following table describes the inbound rule for a security group that enables associated instances to communicate with each other. The rule allows all types of traffic.

Protocol type	Protocol number	Ports	Source IP
-1 (All)	-1 (All)	-1 (All)	The ID of the security group

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/security-group-rules-reference.html

Answer 2

Instances associated with the same security group can’t talk to each other unless you add rules allowing it (with the exception being the default security group). you have to add rules to make them able to communicate.

Answer 3

It depends on the rules. The fact that two or more instances are associated with the same security group is not related to the allowed traffic.

Security groups is a set of allowed traffic rules, while the reference point is the instances themselves (meaning incoming traffic into the instance or outgoing traffic from the instance). The definition whether instances have access to each others depends on the security groups' rules and the network ACLs' rules.

The communication will not be blocked as long there are rules that allow it. A communication can be RDP, ICMP, HTTP/S and more, but it should be allowed in both security groups and NACLs.

A note to remember: by default, AWS blocks ICMP communication (ping), therefore, although the security group may have "All Traffic" allowed rule a ping request will fail if there is no specific rule that allows it.