# The Default Network and Service Names — Docker Compose

Source: https://www.skillbyai.com/en/docker-compose/n-default

> Built-in DNS.

## Reach services by name

Compose creates a default network for each project (named `<project>_default`) and attaches every service. Each service is reachable by its **service name** as a hostname: the API connects to `db:5432`, the web front end calls `http://api:3000`. Container IPs change on every recreation, so always use names. Scaled services (`--scale worker=3`) share one name and DNS returns several addresses.

## Who can talk to whom

Services find each other by name on project networks; custom networks isolate tiers.

![Three ideas: the default network, custom and internal networks, exposure.](assets/figures/docker-compose/section-6-map.svg) — Figure 6.1 — Default network, isolation and exposure.

## An office phone directory

You dial "accounts" and reach whoever is on the accounts desk today; you never need their personal number.

## Use container ports between services

Inside the network use the container port (api:3000), not the published host port.

**Quiz:** How does the api service connect to the database in the demo?

- [ ] By container ID
- [ ] By the host IP and a published port only
- [x] By the hostname db on the project network
- [ ] It cannot connect

*Answer:* By the hostname db on the project network. Service names are DNS names.