Containers are isolated, but they often need to talk to each other and to the outside world. Docker handles this through network drivers.
Network Drivers
Docker supports several network types:
| Driver | Description |
|---|---|
bridge |
Virtual network on the host. Default for all containers |
host |
Container shares the host’s network stack directly |
none |
No network. Complete isolation |
overlay |
For Swarm: network spanning multiple hosts |
docker network ls # list networks
docker network inspect bridge # network details
docker network create my-network # create a network
docker network rm my-network # remove a network
Default Bridge: Containers Communicate by IP
By default every container is attached to the bridge network. Containers can reach each other only by IP address.
# Start two containers
docker run -d --name app1 alpine sleep infinity
docker run -d --name app2 alpine sleep infinity
# Get the IP of the first container
docker inspect app1 | grep IPAddress
# โ 172.17.0.2
# Ping app1 from app2 using its IP
docker exec app2 ping 172.17.0.2
The problem: the IP changes on restart. The solution is a user-defined network.
User-Defined Network: DNS by Container Name
In a user-defined network, Docker automatically registers DNS entries using the container name. Containers can reach each other by name.
# Create a network
docker network create my-app-net
# Start containers on this network
docker run -d --name postgres --network my-app-net postgres:15
docker run -d --name web --network my-app-net my-web-app
# From web, connect to postgres by name:
# postgresql://postgres:5432/db
The container name becomes a reliable address. DNS works automatically.
How Docker Compose Creates a Network
Docker Compose automatically creates a dedicated network for each project. All services in docker-compose.yml are connected to it and can reach each other by service name.
services:
web:
build: .
# Can reach "db" and "redis" by name
db:
image: postgres:15
redis:
image: redis:7
The network is named <project-folder>_default. No extra configuration needed.
To isolate services from each other, declare multiple networks explicitly:
services:
web:
networks:
- frontend
- backend
db:
networks:
- backend # db is unreachable from the frontend network
networks:
frontend:
backend:
Port Mapping: -p HOST:CONTAINER
A container is not reachable from outside by default. The -p flag maps a host port to a container port.
# Syntax: -p HOST:CONTAINER
docker run -p 8080:80 nginx # localhost:8080 โ container:80
docker run -p 127.0.0.1:8080:80 nginx # localhost only
docker run -p 5432:5432 postgres # postgres reachable from host
# Multiple ports
docker run -p 80:80 -p 443:443 nginx
# Random host port
docker run -p 80 nginx # Docker picks the port
docker port <container> # find out which port was assigned
In docker-compose.yml:
ports:
- "8080:80" # host:container
- "127.0.0.1:5432:5432" # localhost only
Example: nginx + app + postgres
A full stack with separate networks for isolation:
version: "3.8"
services:
nginx:
image: nginx:alpine
ports:
- "80:80"
volumes:
- ./nginx.conf:/etc/nginx/nginx.conf:ro
networks:
- frontend
depends_on:
- app
app:
build: .
environment:
DATABASE_URL: postgresql://user:pass@postgres:5432/mydb
networks:
- frontend
- backend
depends_on:
- postgres
postgres:
image: postgres:15
environment:
POSTGRES_USER: user
POSTGRES_PASSWORD: pass
POSTGRES_DB: mydb
volumes:
- pgdata:/var/lib/postgresql/data
networks:
- backend # postgres is not visible from nginx
volumes:
pgdata:
networks:
frontend:
backend:
nginx configuration for proxying to app:
server {
listen 80;
location / {
proxy_pass http://app:8000;
}
}
nginx reaches app by service name. app reaches postgres by service name. postgres is isolated in the backend network and is not accessible from the internet.
Useful Diagnostic Commands
# List all networks
docker network ls
# What is connected to a network
docker network inspect my-app-net
# IP addresses of a container
docker inspect web | grep -A 20 '"Networks"'
# Check connectivity between containers
docker exec app ping db
docker exec app curl http://nginx/health
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