you can make use of the built-in `overlay` driver to enable multi-host communication.

Overlay networks are always created as `attachable`. You can optionally set the [`attachable`](/reference/compose-file/networks.md#attachable) property to `false`.

Consult the [Swarm mode section](/manuals/engine/swarm/_index.md), to see how to set up
a Swarm cluster, and the [Getting started with multi-host networking](/manuals/engine/network/tutorials/overlay.md)
to learn about multi-host overlay networks.

## Specify custom networks

Instead of just using the default app network, you can specify your own networks with the top-level `networks` key. This lets you create more complex topologies and specify [custom network drivers](/engine/extend/plugins_network/) and options. You can also use it to connect services to externally-created networks which aren't managed by Compose.

Each service can specify what networks to connect to with the service-level `networks` key, which is a list of names referencing entries under the top-level `networks` key.

The following example shows a Compose file which defines two custom networks. The `proxy` service is isolated from the `db` service, because they do not share a network in common. Only `app` can talk to both.

```yaml
services:
  proxy:
    build: ./proxy
    networks:
      - frontend
  app:
    build: ./app
    networks:
      - frontend
      - backend
  db:
    image: postgres
    networks:
      - backend

networks:
  frontend:
    # Specify driver options
    driver: bridge
    driver_opts:
      com.docker.network.bridge.host_binding_ipv4: "127.0.0.1"
  backend:
    # Use a custom driver
    driver: custom-driver
```

Networks can be configured with static IP addresses by setting the [ipv4_address and/or ipv6_address](/reference/compose-file/services.md#ipv4_address-ipv6_address) for each attached network.

Networks can also be given a [custom name](/reference/compose-file/networks.md#name):

```yaml
services:
  # ...
networks:
  frontend:
    name: custom_frontend
    driver: custom-driver-1
```

## Configure the default network

Instead of, or as well as, specifying your own networks, you can also change the settings of the app-wide default network by defining an entry under `networks` named `default`:

```yaml
services:
  web:
    build: .
    ports:
      - "8000:8000"
  db:
    image: postgres

networks:
  default:
    # Use a custom driver
    driver: custom-driver-1
```

## Use a pre-existing network

If you want your containers to join a pre-existing network, use the [`external` option](/reference/compose-file/networks.md#external)
```yaml
services:
  # ...
networks:
  network1:
    name: my-pre-existing-network
    external: true
```

Instead of attempting to create a network called `[projectname]_default`, Compose looks for a network called `my-pre-existing-network` and connects your app's containers to it.

## Further reference information 

For full details of the network configuration options available, see the following references:

- [Top-level `networks` element](/reference/compose-file/networks.md)
- [Service-level `networks` attribute](/reference/compose-file/services.md#networks)