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Docker Intermediate: Images and Compose That Survive Real Projects
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Docker Intermediate: Images and Compose That Survive Real Projects

Check Yourself: Networks Beyond Defaults

Five scenario questions on name lookup, localhost, aliases, published ports and EXPOSE. If you miss more than one, reread the lessons in this section.

5 min DockerSecurityDeployment and ContainersPackaging and Dependencies Intermediate
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1. Two containers are on the default bridge network. One asks for http://plain-web and gets bad address. What explains it?
Names only work if you also publish a port. The web server is not running. The containers must be started in a certain order. The default bridge has no name lookup. Create your own network and attach both containers to it.
2. Your app container uses the address http://localhost:5432 to reach the database container and gets connection refused. The database is healthy. What is wrong?
The database needs a restart. Port 5432 is blocked by Windows Firewall. Inside a container, localhost is the container itself. Use the database container's name on a shared network. localhost works only on Linux.
3. Two containers are started on one network with the same --network-alias api. A third container looks up api. What happens?
Only the container that started first answers, always. The lookup returns the addresses of both containers, which spreads requests but does not check whether either one is healthy. Docker refuses the second container because the alias is taken. The lookup fails because aliases need a published port.
4. You run docker run -p 5432:5432 for a development database with a default password. What is the problem, and what is the better command?
There is no problem, because Docker blocks outside traffic by default. -p only works with 127.0.0.1, so your command fails. The port number is wrong, because databases cannot use 5432 in a container. The port is published on every network interface, so other computers on your network can reach it. Use -p 127.0.0.1:5432:5432, or publish nothing and let other containers connect by name.
5. A Dockerfile has EXPOSE 8080. You start the container with docker run and no -p option. Can your browser reach http://localhost:8080?
Yes, after a few seconds, when Docker finishes starting. No. EXPOSE only documents the port, and nothing is published until you use -p or -P. Yes, but only from Chrome. Yes, EXPOSE opens the port on your computer.

About

Five scenario questions on name lookup, localhost, aliases, published ports and EXPOSE. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
08/01/2026
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