Why can't containers see each other without a network?

#Why this happens

Containers can't see each other without a network because Docker isolates them by default - each container starts with its own network space, with no shared interface and no routes to other containers.

#Key reasons

1. Network isolation is a security foundation A container shouldn't automatically have access to other applications. Without an explicit network, Docker doesn't link them together.

2. No shared network space A container has its own network namespace. If they aren't joined, they have no shared interfaces or ways to route traffic.

3. No DNS name resolution Docker creates an internal DNS only within a single network. Without a network, containers don't know each other's names and can't find their neighbor.

#Summary

So without a network, they're just independent applications, each "in its own world." For them to communicate, they need to be placed in the same Docker network.