Bridge network default behavior in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the default network driver used by Docker when no network is specified?

Docker uses the bridge network driver by default. It creates a private internal network on the host for containers to communicate.

beginner

How does Docker's default bridge network handle container IP addressing?

Docker assigns each container a unique IP address within the bridge network's subnet, allowing containers to communicate using these IPs.

intermediate

Can containers on the default bridge network communicate with each other by container name?

No, containers on the default bridge network cannot resolve each other by name. They must use IP addresses or user-defined networks for name resolution.

intermediate

What is the role of NAT in Docker's default bridge network?

Docker uses Network Address Translation (NAT) to allow containers on the bridge network to access external networks like the internet through the host.

beginner

How can you inspect the default bridge network configuration in Docker?

Use the command docker network inspect bridge to see details like subnet, gateway, and connected containers.

What is the default subnet used by Docker's bridge network?

A 172.17.0.0/16

B 192.168.0.0/24

C 10.0.0.0/8

D 127.0.0.1/32

Docker's default bridge network typically uses the subnet 172.17.0.0/16 for container IP addressing.

Can containers on the default bridge network communicate with each other using container names?

A Yes, by default

B Only on Windows hosts

C No, they must use IP addresses

D Only if linked explicitly

Containers on the default bridge network cannot resolve each other by name; they must use IP addresses or user-defined networks.

What command shows detailed info about the default bridge network?

A docker network ls

B docker inspect container

C docker ps -a

D docker network inspect bridge

The command 'docker network inspect bridge' displays detailed configuration of the default bridge network.

How does Docker allow containers on the bridge network to access the internet?

A Using NAT through the host

B Direct public IP assignment

C VPN tunneling

D Containers cannot access the internet

Docker uses Network Address Translation (NAT) on the host to enable containers to access external networks.

Which of these is true about the default Docker bridge network?

A It assigns public IP addresses to containers

B It isolates containers from the host network

C It allows automatic DNS resolution by container name

D It requires manual IP assignment

The default bridge network isolates containers from the host network using a private subnet and NAT.

Explain how Docker's default bridge network manages container communication and external access.

Think about how containers talk to each other and the outside world.

Describe how to inspect and understand the configuration of Docker's default bridge network.

Use Docker commands to explore network details.

Practice

1. What is the default behavior of Docker's bridge network regarding container communication?

easy

A. Containers can communicate by container name without extra setup.

B. Containers cannot communicate with each other at all.

C. Containers can communicate with each other by IP address only.

D. Containers share the host network directly.

Solution

1. Step 1: Understand bridge network basics

   The default bridge network connects containers on the same Docker host and allows communication by IP address.

2. Step 2: Analyze name resolution behavior

   By default, containers on the bridge network cannot resolve each other's names without a custom network.

3. Final Answer:

   Containers can communicate with each other by IP address only. -> Option C

4. Quick Check:

   Bridge network communication = IP only [OK]

Hint: Default bridge allows IP communication, not name resolution [OK]

Common Mistakes:

• Assuming container names resolve by default
• Thinking containers share host network directly
• Believing containers cannot communicate at all

2. Which Docker command shows the default bridge network details?

easy

A. docker info bridge

B. docker network ls

C. docker container ls

D. docker network inspect bridge

Solution

1. Step 1: Identify command to inspect networks

   To see details of a specific network, use docker network inspect <network-name>.

2. Step 2: Apply to default bridge network

   The default bridge network is named "bridge", so the command is docker network inspect bridge.

3. Final Answer:

   docker network inspect bridge -> Option D

4. Quick Check:

   Inspect bridge network = docker network inspect bridge [OK]

Hint: Inspect networks with 'docker network inspect <name>' [OK]

Common Mistakes:

• Using 'docker container ls' to see networks
• Trying 'docker info bridge' which is invalid
• Confusing 'docker network ls' with inspect

3. Given two containers on the default bridge network, what happens if you try to ping one container by its name from the other?

medium

A. Ping fails because default bridge does not support container name resolution.

B. Ping fails unless containers are on a custom network.

C. Ping succeeds only if containers share the same image.

D. Ping succeeds because default bridge supports name resolution.

Solution

1. Step 1: Recall default bridge network name resolution

   The default bridge network does not provide automatic DNS resolution for container names.

2. Step 2: Understand ping behavior by name

   Trying to ping by container name will fail because the name cannot be resolved to an IP address.

3. Final Answer:

   Ping fails because default bridge does not support container name resolution. -> Option A

4. Quick Check:

   Default bridge name resolution = no [OK]

Hint: Default bridge lacks DNS for container names [OK]

Common Mistakes:

• Assuming ping by name works by default
• Confusing image sharing with network name resolution
• Thinking custom networks are not needed

4. You created two containers on the default bridge network and tried to connect them by name, but it failed. What is the likely cause?

medium

A. The containers are not running.

B. You forgot to create a user-defined bridge network.

C. The Docker daemon is not started.

D. You used the wrong container names.

Solution

1. Step 1: Understand default bridge limitations

   Default bridge network does not support container name resolution for communication.

2. Step 2: Identify solution for name-based communication

   Creating a user-defined bridge network enables automatic DNS resolution of container names.

3. Final Answer:

   You forgot to create a user-defined bridge network. -> Option B

4. Quick Check:

   Name resolution needs user-defined bridge [OK]

Hint: Use user-defined bridge for name resolution [OK]

Common Mistakes:

• Blaming container state without checking network
• Assuming Docker daemon is stopped without evidence
• Ignoring network type for name resolution

5. You want two containers to communicate by name and also access the internet. Which approach using Docker's networking should you take?

hard

A. Connect both containers to a user-defined bridge network which supports name resolution and NAT for internet access.

B. Use the default bridge network and manually edit /etc/hosts in each container.

C. Connect containers to the host network to share internet and name resolution.

D. Create a macvlan network to enable name resolution and internet access.

Solution

1. Step 1: Identify network supporting name resolution and NAT

   User-defined bridge networks provide automatic DNS for container names and NAT for internet access.

2. Step 2: Evaluate other options

   Default bridge lacks name resolution; host network shares host IP but no DNS; macvlan isolates containers and does not provide NAT by default.

3. Final Answer:

   Connect both containers to a user-defined bridge network which supports name resolution and NAT for internet access. -> Option A

4. Quick Check:

   User-defined bridge = name resolution + NAT [OK]

Hint: User-defined bridge = name resolution + internet [OK]

Common Mistakes:

• Editing /etc/hosts manually is error-prone
• Assuming host network provides container DNS
• Using macvlan without NAT causes no internet