Cheat SheetsDockerNetworking

Networking — Cheat Sheet

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Cheat Sheet · AiCanCode.org
Networking
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Docker Networking

Docker provides isolated virtual networks for containers. Understanding bridge, host, and overlay networks — and how containers discover each other by name — is essential for multi-container apps.

  • ✓User-defined bridge networks provide automatic DNS: containers resolve each other by name.
  • ✓Default bridge network only allows IP-based communication.
  • ✓Port mapping (-p host:container) is required to expose container services externally.
  • ✓Host network mode removes network isolation — container uses host's network stack directly.
  • ✓Overlay networks connect containers across multiple hosts (Swarm/Kubernetes).
  • ✓Docker Compose automatically creates a user-defined network for all services in the file.
bash — user-defined bridge network
# Default bridge: containers communicate by IP only (fragile)

docker network ls

# NETWORK ID   NAME      DRIVER   SCOPE

# 3b8c9f1a2d   bridge    bridge   local   ← default

# 8e4f7c1b3d   host      host     local

# 9a2b5e0c8f   none      null     local



# ✅ Create user-defined bridge (containers get DNS by name)

docker network create my-app-net



# Run containers on the same network

docker run -d --name postgres --network my-app-net postgres:16

docker run -d --name backend  --network my-app-net \

  -e DATABASE_URL=postgres://postgres:5432/mydb \   # resolve 'postgres' by name!

  my-backend:latest



# Test DNS resolution inside a container

docker exec -it backend sh

# Inside: ping postgres   → resolves to postgres container IP
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