User-defined bridges provide automatic DNS resolution between containers.
Containers on the default bridge network can only access each other by IP addresses, unless you use the --link option, which is considered legacy. On a user-defined bridge network, containers can resolve each other by name or alias.
Imagine an application with a web front-end and a database back-end. If you call your containers web and db, the web container can connect to the db container at db, no matter which Docker host the application stack is running on.
If you run the same application stack on the default bridge network, you need to manually create links between the containers (using the legacy --link flag). These links need to be created in both directions, so you can see this gets complex with more than two containers which need to communicate. Alternatively, you can manipulate the /etc/hosts files within the containers, but this creates problems that are difficult to debug.
User-defined bridges provide better isolation.
All containers without a --network specified, are attached to the default bridge network. This can be a risk, as unrelated stacks/services/containers are then able to communicate.
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. This type of variable sharing is not 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 and configs.
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.