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Darshan Vasani
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🧭 Docker Port Mapping & CLI Flag CheatSheet

#webdev #docker #javascript #devops

🧭 Docker Port Mapping & CLI Flag CheatSheet

🚪 Port Mapping 101: -p vs -P

Flag Meaning Analogy Example
-p <host>:<container> Map host port to container port Custom Door Mapping 🚪 -p 8080:3000
-P (uppercase) Auto-map all exposed ports to random host ports 🔀 Auto Door Mapping -P

🎯 Example 1: Manual Mapping with -p 

docker run -p 8080:3000 my-app
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  • 🔧 Host port 8080 mapped to container's 3000
  • Access via localhost:8080

🎲 Example 2: Auto Mapping with -P 

docker run -P my-app
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  • Maps ALL EXPOSEd ports to random available host ports
  • View with docker ps

🌍 Multiple Port Mappings 

docker run -p 3000:3000 -p 5000:5000 my-multi-app
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  • Maps multiple services/APIs or frontend/backend apps
  • Great for full-stack containers!

📡 Exposing Ports in Dockerfile 

EXPOSE 3000 5000 7000
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🧠 Note:
EXPOSE is just documentation for which ports the container listens to.
It does not publish or map ports. Use -p or -P in docker run to do that.

📦 Exposing a Range of Ports 

docker run -p 8000-8010:8000-8010 my-app
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  • Useful for services like WebRTC, game servers, or load balancers needing multiple ports.
  • 🔄 Keep range symmetrical between host and container.

🧪 Combine with --rm, -it, -d

Flag Meaning Emoji Use-case
--rm Auto-remove container when it exits 🧹 Clean test runs
-i Interactive (stdin open) 🎤 Needed for terminal apps
-t TTY (format output) 🖥️ Pretty output formatting
-it Combo: interactive + tty 🧑‍💻 Needed for shell, REPLs
-d Detached mode (run in background) 🛸 Long-running servers

🔥 Full Command Example 

docker run -it --rm -p 4000:3000 my-app
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  • Runs interactively
  • Auto deletes after exit
  • Maps port 3000 → 4000

🚀 Run in Detached Mode + Multiple Ports + Clean Exit 

docker run -d --rm -p 3000:3000 -p 5000:5000 my-app
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  • Runs in background
  • Auto-cleans after stopping
  • Maps frontend + backend

🕵️‍♂️ How to Check Mapped Ports? 

docker ps
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Column What it Shows
PORTS Host:Container mapping (e.g., 0.0.0.0:8080->3000/tcp)

🛑 Stop a Detached Container 

docker stop <container_id>
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🧠 Real-World Analogies

Concept Analogy
Container port 📦 Internal phone extension
Host port ☎️ Public phone number
-p Assigning specific number to extension
-P Letting system pick a number for you randomly

✅ Summary Cheatsheet Table

Option Purpose Usage
-p 8080:80 Manual port map host:container
-p 8000-8010:8000-8010 Port range mapping Bulk apps
-P Auto-map all exposed ports Quick tests
EXPOSE 3000 Doc port in Dockerfile Doesn't publish
--rm Auto-remove after exit Clean containers
-it Interactive terminal Needed for shell
-d Run in background Daemonized apps

🧾 Real Full Example: Production-style Run 

docker run -d --rm \
  -p 8080:80 \
  -p 443:443 \
  --name my-nginx \
  nginx:alpine
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✅ Starts Nginx with HTTP & HTTPS
✅ Cleans itself after docker stop
✅ Runs in background with custom name

🧠 Bonus Tip: Use --network to map ports between containers!

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