Docker containers starts at different subtnets - wordpress

Question

I have to start up 3 containers, nginx, wordpress and wordpress2. The problem is the nginx container starts at 172.17.0.2, wordpress at 172.18.0.3 and wordpress2 at 172.19.0.4.
The wordpress containers starts with their docker-compose.yml configured as i show:
For wordpress:
version: '2'
services:
db:
image: mysql:5.7
volumes:
- "./.data/db:/var/lib/mysql"
restart: always
environment:
MYSQL_ROOT_PASSWORD: wordpress
MYSQL_DATABASE: wordpress
MYSQL_USER: wordpress
MYSQL_PASSWORD: wordpress
wordpress:
depends_on:
- db
image: wordpress:latest
links:
- db
ports:
- "8000:80"
networks:
- ipv4_address: 172.17.0.5
restart: always
environment:
WORDPRESS_DB_HOST: db:3306
WORDPRESS_DB_PASSWORD: wordpress
For wordpress2:
version: '2'
services:
db:
image: mysql:5.7
volumes:
- "./.data/db:/var/lib/mysql"
restart: always
environment:
MYSQL_ROOT_PASSWORD: wordpress
MYSQL_DATABASE: wordpress
MYSQL_USER: wordpress
MYSQL_PASSWORD: wordpress
wordpress:
depends_on:
- db
image: wordpress:latest
links:
- db
ports:
- "8001:80"
networks:
- ipv4_address: 172.17.0.6
restart: always
environment:
WORDPRESS_DB_HOST: db:3306
WORDPRESS_DB_PASSWORD: wordpress
As you can see, i put at networks field the ipv4_address but when i start up the docker-compose.yml it shows error at this line. I thought that by default, all containers usually stars at same network but not in this case. Can you help me to put all containers at same subnet?

Answer 1

Each docker-compose.yml will create it's own subnet. So your 3 containers can't see each other.
Networks created by other docker-compose are said to be external so you have to declare them as such. You can see these networks using docker network ls.
After declaring the external networks in your external networks in your docker-compose.yml file, you will have to define what networks should be using by the nginx service. You should keep the default network.
Usually the default name for your docker-compose network is <current_dir>_default. So suppose your current directory is called yourproject1 the network name is going to be yourproject1_default.
And then you will be able to access your wordpress containers using external_links (better than ip addresses IMO)
Here what I would do for nginx's docker-compose.yml file to make him able to see wordpress containers :
version: '2'
services:
nginx:
...
external_links:
- your_project1_wordpress1_1:wordpress1
- your_other_project2_wordpress2_1:wordpress2
networks:
- default
- your_project1_default
- your_other_project2_defaul
networks:
your_project1_default:
external: true
your_other_project2_default:
external: true