How to fix Network mynetwork declared as external, but could not be found in Docker

First, check what networks currently exist on your Docker host:

docker network ls

Look for the network name mentioned in the error. If it's not listed, you'll need to create it. Pay attention to the exact spelling and case - network names are case-sensitive.

You can also filter for a specific name:

docker network ls --filter name=mynetwork

Create the network that your docker-compose.yml expects:

docker network create mynetwork

For a bridge network (the default), this is all you need. If you need specific options:

# Create with a specific subnet
docker network create --subnet=172.28.0.0/16 mynetwork

# Create with a specific driver
docker network create --driver bridge mynetwork

After creating the network, run docker-compose up again.

Check your docker-compose.yml to ensure the network name matches exactly:

networks:
  mynetwork:
    external: true

If you're using the name property, verify it matches the created network:

networks:
  app_network:
    name: mynetwork
    external: true

In this case, the internal reference is app_network, but Docker looks for a network named mynetwork.

Use docker compose config to see the resolved configuration including any variable substitutions:

docker compose config

If you don't actually need to share this network with other projects or manually-created containers, simply remove external: true:

Before:

networks:
  mynetwork:
    external: true

After:

networks:
  mynetwork:
    driver: bridge

Or remove the networks section entirely - Compose will create a default network automatically:

services:
  web:
    image: nginx
  db:
    image: postgres
# No networks section needed - Compose creates a default network

Since Docker Compose version 3.5+, you can use the name property to control the exact network name without the project prefix:

networks:
  shared:
    name: shared_network
    driver: bridge

This creates a network called exactly shared_network (not projectname_shared_network). When you need to share it across projects, other Compose files can reference it as external:

networks:
  shared:
    name: shared_network
    external: true

This approach avoids needing to manually create networks in most cases - the first Compose project to start will create it.

When multiple Compose projects need to communicate, create a shared external network:

# Create the shared network once
docker network create shared_net

Then in each docker-compose.yml:

services:
  app:
    image: myapp
    networks:
      - shared_net
      - default  # Also keep the default network for internal services

networks:
  shared_net:
    external: true

Services on the shared network can reach each other using their service names as hostnames.