Docker creates virtual networks. Containers on the same network can reach each other by container name. The default bridge network connects containers on the same host; host network removes isolation; overlay spans multiple hosts.

In Docker Compose, each service gets a DNS entry matching its service name - a web service can reach a database service at db:5432 without knowing the IP. Port mapping ( -p 8080:80 ) exposes a container port to the host. User-defined bridge networks provide automatic DNS resolution; the default bridge does not. Overlay networks allow cross-host container communication in Docker Swarm.