200-901 Application Deployment and Security Practice Question
In a Docker bridge network, two containers can communicate with each other using which identifier by default?
⚠ Common exam trap
The trap is assuming container names always resolve; candidates must remember that name resolution only works on user-defined bridge networks, not the default bridge.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Their IP addresses only
In a Docker bridge network, containers can reach each other by IP address by default because the embedded DNS resolver only provides name resolution for user-defined bridge networks, not the default bridge. On the default bridge, only IP-based communication works unless you use legacy --link.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Their IP addresses only
Why this is correct
Docker's default bridge network provides no automatic DNS resolution between containers, so each container must address the other by its assigned IP address. Container names resolve only on user-defined bridge networks, making IP addresses the sole default identifier here.
- ✗
Their image names
Why it's wrong here
Image names identify the template used to create containers, not running instances, so no DNS record or address maps to them. They are tempting because image names appear in docker ps output and deployment manifests, but they are the correct identifier only when pulling or building images, never for inter-container traffic.
- ✗
Their container names
Why it's wrong here
Docker's embedded DNS resolves container names only on user-defined bridge networks; the default bridge network provides no automatic name resolution, so containers must use IP addresses or legacy --link. Container names are tempting because they work on custom networks, which is where name-based discovery is the intended mechanism.
- ✗
Their MAC addresses
Why it's wrong here
Docker's embedded DNS resolves container names on a user-defined bridge network, so containers communicate by name or alias, not MAC address. MAC addressing is tempting because Layer 2 forwarding uses it, but Docker resolves names to container IPs. MAC addresses change per container lifecycle and are not the default identifier.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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