Fix Docker Compose Network Failed

What's Actually Happening

Docker Compose services cannot communicate with each other. Network connectivity between containers fails.

The Error You'll See

```bash $ docker-compose up

ERROR: Network myapp_default declared as external, but could not be found ```

Or containers cannot ping each other:

$ docker exec app ping db
ping: bad address 'db'

Why This Happens

1. 1.Network not created - Network missing
2. 2.IP conflict - Overlapping IP ranges
3. 3.DNS resolution - Container names not resolving
4. 4.Firewall rules - Blocking container traffic

Step 1: Check Network Status

```bash # List networks: docker network ls

# Inspect network: docker network inspect myapp_default

# Check container IPs: docker inspect app | grep IPAddress ```

Step 2: Create Network

```bash # Create network: docker network create myapp_default

# Or in docker-compose.yml: networks: default: driver: bridge ```

Step 3: Fix IP Conflicts

# In docker-compose.yml:
networks:
  appnet:
    ipam:
      config:
        - subnet: 172.20.0.0/16

Step 4: Recreate Network

```bash # Remove network: docker network rm myapp_default

# Recreate: docker-compose up -d ```

Docker Compose Network Checklist

Verify the Fix

docker exec app ping db
# Output: 64 bytes from db: seq=0 ttl=64 time=0.1ms

Related Issues

• [Fix Docker Container Cannot Connect](/articles/fix-docker-container-cannot-connect)
• [Fix Docker DNS Resolution Failed](/articles/fix-docker-dns-resolution-failed)