Having issues with communications between EC2 vms on the same subnet

Question

I have an issue with EC2 vms not able to communicate with each other and hoping perhaps you can assist.

-	I have 4 instances all on the same subnet and vpc.

-	I have all traffic allowed to the subnet range of /20 on both inbound and outbound with regards to the security groups

-	EC2 B can be pinged by all but A, C and D can’t be pinged by B or each other

-	Can’t telnet to listening port on B either and FW port is opened to subnet. (All Windows OS btw)

Things I’ve tried:

I started with more stricter rules (sorta) but using a /16 instead of a /20 but for specific ports however forwarding of logs was not working. And I couldn’t ping anything not even EC2 B.

What am I missing?

Answer

Have you altered the NACL for the subnet?  The default should allow all traffic but if you have altered them, check that.
Have you checked the OS level for firewall settings?

Answer

You may want to check the Routing table(s) you are using in this VPC and subnets. If you have the original default VPC and main routing table to compare settings, look for a route target with 'local' Target and correct destination CIDR block for your subnet. Also check each subnet for similar Route table settings for local traffic.

There is a new feature in AWS Console for VPC which displays a Resource map of your VPC, might give clues to whatever is missing or misconfigured.

Having issues with communications between EC2 vms on the same subnet

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