Bridge network default behavior in Docker - Step-by-Step Execution

Process Flow - Bridge network default behavior

Docker daemon starts

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Create default bridge network

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Container connects to bridge

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Assign container IP from bridge subnet

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Enable container communication via bridge

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Enable NAT for external access

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Container can communicate internally and externally

Docker creates a default bridge network at startup. Containers connected to it get IPs and can talk to each other and outside via NAT.

Execution Sample

Docker

docker network ls
docker run -d --name c1 nginx
docker inspect c1
ping <container_ip>

List networks, run a container on default bridge, inspect IP, and ping to test connectivity.

Process Table

Step	Action	Result	Network State
1	Docker daemon starts	Default bridge network 'bridge' created	bridge network exists with subnet 172.17.0.0/16
2	Run container c1 without network option	Container connected to default bridge	c1 assigned IP 172.17.0.2
3	Inspect container c1	Shows IP 172.17.0.2 and bridge network	bridge network has 1 container connected
4	Run container c2	Connected to default bridge	c2 assigned IP 172.17.0.3
5	Ping c2 from c1	Ping successful	Containers communicate internally via bridge
6	Container c1 accesses internet	Access allowed via NAT	Bridge network performs NAT for outbound traffic
7	Stop container c1	Container removed from bridge	bridge network has 1 container connected
8	Remove container c2	No containers connected	bridge network empty
9	Exit	No more containers connected	bridge network remains available

💡 No more containers connected, default bridge network remains active for future use

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 7	After Step 8
bridge network	Created with subnet 172.17.0.0/16	1 container connected (c1)	2 containers connected (c1, c2)	1 container connected (c2)	0 containers connected

Key Moments - 3 Insights

Why does the container get an IP address automatically?

Because the container connects to the default bridge network which assigns IPs from its subnet automatically, as shown in execution_table step 2 and 4.

Can containers on the default bridge communicate with each other?

Yes, containers connected to the default bridge can communicate internally, demonstrated by the successful ping in step 5.

How does a container access the internet if connected to the default bridge?

The default bridge network uses NAT (Network Address Translation) to allow containers to access external networks, shown in step 6.

Visual Quiz - 3 Questions

Look at the execution table, what IP address is assigned to container c1 after step 2?

A 172.17.0.2

B 172.17.0.3

C 192.168.1.2

D 10.0.0.2

💡 Hint

Check the 'Result' column in row for step 2 in the execution_table.

At which step do containers first communicate with each other?

A Step 3

B Step 5

C Step 6

D Step 4

💡 Hint

Look for the 'Ping successful' action in the execution_table.

If container c1 is removed, how many containers remain connected to the bridge network?

A 0

B 2

C 1

D None, network is deleted

💡 Hint

Check the 'Network State' column after step 7 in the execution_table.

Concept Snapshot

Docker default bridge network:
- Created automatically at Docker start
- Assigns IPs to containers from 172.17.0.0/16
- Enables container-to-container communication
- Provides NAT for internet access
- Network persists even if no containers connected

Full Transcript

When Docker starts, it creates a default bridge network named 'bridge'. Containers run without specifying a network connect to this bridge automatically. Each container gets an IP address from the bridge subnet, typically 172.17.0.x. Containers connected to this bridge can communicate with each other directly. The bridge network also provides NAT so containers can access the internet. Even if all containers stop or are removed, the default bridge network remains available for future containers.