> If the maximum transmission unit (MTU) between hosts in your subnets is different, or your instances communicate with peers over the internet, you must add th

> If the maximum transmission unit (MTU) between hosts in your subnets is different, or your instances communicate with peers over the internet, you must add the following network ACL rule, both inbound and outbound. https://docs.aws.amazon.com/vpc/latest/userguide/vpc-network-acls.html#path_mtu_discovery https://docs.aws.amazon.com/vpc/latest/userguide/vpc-network... If your instances communicate with peers over the internet. So by default, AWS was built to assume your instances do not communicate over the internet, lol.