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2nd chunk of `content/manuals/engine/network/drivers/bridge.md`
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  Using a user-defined network provides a scoped network in which only containers attached to that network are able to communicate.

- **Containers can be attached and detached from user-defined networks on the fly**.

  During a container's lifetime, you can connect or disconnect it from
  user-defined networks on the fly. To remove a container from the default
  bridge network, you need to stop the container and recreate it with different
  network options.

- **Each user-defined network creates a configurable bridge**.

  If your containers use the default bridge network, you can configure it, but
  all the containers use the same settings, such as MTU and `iptables` rules.
  In addition, configuring the default bridge network happens outside of Docker
  itself, and requires a restart of Docker.

  User-defined bridge networks are created and configured using
  `docker network create`. If different groups of applications have different
  network requirements, you can configure each user-defined bridge separately,
  as you create it.

- **Linked containers on the default bridge network share environment variables**.

  Originally, the only way to share environment variables between two containers
  was to link them using the [`--link` flag](../links.md). This type of
  variable sharing isn't possible with user-defined networks. However, there
  are superior ways to share environment variables. A few ideas:

  - Multiple containers can mount a file or directory containing the shared
    information, using a Docker volume.

  - Multiple containers can be started together using `docker-compose` and the
    compose file can define the shared variables.

  - You can use swarm services instead of standalone containers, and take
    advantage of shared [secrets](/manuals/engine/swarm/secrets.md) and
    [configs](/manuals/engine/swarm/configs.md).

Containers connected to the same user-defined bridge network effectively expose all ports
to each other. For a port to be accessible to containers or non-Docker hosts on
different networks, that port must be _published_ using the `-p` or `--publish`
flag.

## Options

The following table describes the driver-specific options that you can pass to
`--opt` when creating a custom network using the `bridge` driver.

| Option                                                                                          | Default                     | Description                                                                                         |
|-------------------------------------------------------------------------------------------------|-----------------------------|-----------------------------------------------------------------------------------------------------|
| `com.docker.network.bridge.name`                                                                |                             | Interface name to use when creating the Linux bridge.                                               |
| `com.docker.network.bridge.enable_ip_masquerade`                                                | `true`                      | Enable IP masquerading.                                                                             |
| `com.docker.network.bridge.gateway_mode_ipv4`<br/>`com.docker.network.bridge.gateway_mode_ipv6` | `nat`                       | Control external connectivity. See [Packet filtering and firewalls](packet-filtering-firewalls.md). |
| `com.docker.network.bridge.enable_icc`                                                          | `true`                      | Enable or Disable inter-container connectivity.                                                     |
| `com.docker.network.bridge.host_binding_ipv4`                                                   | all IPv4 and IPv6 addresses | Default IP when binding container ports.                                                            |
| `com.docker.network.driver.mtu`                                                                 | `0` (no limit)              | Set the containers network Maximum Transmission Unit (MTU).                                         |
Title: Features of User-Defined Bridge Networks and Options
Summary
This section details advantages of user-defined bridge networks in Docker, like dynamic container attachment/detachment and configurable bridges. It also mentions how linked containers on the default bridge share environment variables, but user-defined networks require alternative sharing methods like volumes or Docker Compose. Containers on the same user-defined bridge expose all ports internally, with external access requiring port publishing. The section then lists driver-specific options for creating custom bridge networks, including interface name, IP masquerading, gateway mode, inter-container connectivity, host binding IP, and MTU setting.