This command limits container memory usage to 512 MB and defines the CPU quota of 0.5 for half a core.

> **Monitor the real-time resource usage**
>
> You can use the `docker stats` command to monitor the real-time resource usage of running containers. This helps you understand whether the allocated resources are sufficient or need adjustment.

By effectively using these `docker run` flags, you can tailor your containerized application's behavior to fit your specific requirements.

## Try it out

In this hands-on guide, you'll see how to use the `docker run` command to override the container defaults.

1. [Download and install](/get-started/get-docker/) Docker Desktop.

### Run multiple instances of the Postgres database

1.  Start a container using the [Postgres image](https://hub.docker.com/_/postgres) with the following command:
    
    ```console
    $ docker run -d -e POSTGRES_PASSWORD=secret -p 5432:5432 postgres
    ```

    This will start the Postgres database in the background, listening on the standard container port `5432` and mapped to port `5432` on the host machine.

2. Start a second Postgres container mapped to a different port. 

    ```console
    $ docker run -d -e POSTGRES_PASSWORD=secret -p 5433:5432 postgres
    ```

    This will start another Postgres container in the background, listening on the standard postgres port `5432` in the container, but mapped to port `5433` on the host machine. You override the host port just to ensure that this new container doesn't conflict with the existing running container.

3. Verify that both containers are running by going to the **Containers** view in the Docker Desktop Dashboard.

    ![A screenshot of the Docker Desktop Dashboard showing the running instances of Postgres containers](images/running-postgres-containers.webp?border=true)

### Run Postgres container in a controlled network

By default, containers automatically connect to a special network called a bridge network when you run them. This bridge network acts like a virtual bridge, allowing containers on the same host to communicate with each other while keeping them isolated from the outside world and other hosts. It's a convenient starting point for most container interactions. However, for specific scenarios, you might want more control over the network configuration.

Here's where the custom network comes in. You create a custom network by passing `--network` flag with the `docker run` command. All containers without a `--network` flag are attached to the default bridge network.

Follow the steps to see how to connect a Postgres container to a custom network.

1. Create a new custom network by using the following command:

    ```console
    $ docker network create mynetwork
    ```

2. Verify the network by running the following command:

    ```console
    $ docker network ls
    ```

    This command lists all networks, including the newly created "mynetwork".

3. Connect Postgres to the custom network by using the following command:

    ```console
    $ docker run -d -e POSTGRES_PASSWORD=secret -p 5434:5432 --network mynetwork postgres
    ```

    This will start Postgres container in the background, mapped to the host port 5434 and attached to the `mynetwork` network. You passed the `--network` parameter to override the container default by connecting the container to custom Docker network for better isolation and communication with other containers. You can use `docker network inspect` command to see if the container is tied to this new bridge network.


    > **Key difference between default bridge and custom networks**
    >
    > 1. DNS resolution: By default, containers connected to the default bridge network can communicate with each other, but only by IP address. (unless you use `--link` option which is considered legacy). It is not recommended for production use due to the various [technical shortcomings](/engine/network/drivers/bridge/#differences-between-user-defined-bridges-and-the-default-bridge). On a custom network, containers can resolve each other by name or alias.