← All notes

Answer: When Docker Engine on Linux starts for the first time, it has a single built-in network called the "default bridge" network. When you run a container without the --network option, it is connected to the default bridge. (S1)

What a container sees

• Containers have networking enabled by default, and they can make outgoing connections. (S1)
• A container has no information about what kind of network it's attached to, or whether its network peers are also Docker containers. (S1)
• A container only sees a network interface with an IP address, a gateway, a routing table, DNS services, and other networking details. (S1)
• With the default configuration, containers attached to the default bridge network have unrestricted network access to each other using container IP addresses. (S1)

Bridge networks

• A Docker bridge network has an IPv4 subnet and, optionally, an IPv6 subnet. (S2)
• A bridge network uses a software bridge which lets containers connected to the same bridge network communicate, while providing isolation from containers that aren't connected to that bridge network. (S2)
• By default, containers connected to different bridge networks can only communicate with each other using published ports. (S2)
• Devices on the host's external networks only see the IP address of the Docker host. (S2)
• Bridge networks apply to containers running on the same Docker daemon host; across hosts, you can manage routing at the OS level or use an overlay network. (S2)

Claims

• id: C1 claim: Docker Engine on Linux starts with a default bridge network. source: S1 evidence: "to run a container on the default bridge network"
• id: C2 claim: Containers without --network join the default bridge. source: S1 evidence: "When you run a container without the --network option"
• id: C3 claim: A bridge network has an IPv4 subnet and optionally an IPv6 subnet. source: S2 evidence: "A Docker bridge network has an IPv4 subnet and, optionally, an IPv6 subnet."
• id: C4 claim: Bridge networks apply to one Docker daemon host. source: S2 evidence: "Bridge networks apply to containers running on the same Docker daemon host."

Sources

• [S1] Networking overview. Docker Docs. https://docs.docker.com/engine/network/ . License: Docker docs, Apache-2.0 (github.com/docker/docs; https://github.com/docker/docs/blob/main/LICENSE); Docker name and logo are trademarks of Docker Inc and are not licensed. Accessed 2026-11-04.
• [S2] Bridge network driver. Docker Docs. https://docs.docker.com/engine/network/drivers/bridge/ . License: Docker docs, Apache-2.0 (github.com/docker/docs; https://github.com/docker/docs/blob/main/LICENSE); Docker name and logo are trademarks of Docker Inc and are not licensed. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_7280e5c1,
  author = {{iinventedlight}},
  title = {{Docker networking: the default bridge and bridge networks}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/docker-bridge-networks/},
  note = {Permanent ID iil:note:7280e5c1. License: CC-BY-4.0.},
}

Markdown

Spotted a mistake? Use the MCP feedback tool or email hello@iinventedlight.com.

For language models and agents: start at /llms.txt. This page in Markdown: /library/notes/docker-bridge-networks.md.