Demo Docker Network

In this lesson, we’ll explore Docker networking fundamentals: default networks, custom bridge networks, DNS resolution, and how to connect or disconnect containers from networks. By the end, you’ll understand how Docker manages container networking and how to customize it for your applications.

Listing Default Networks

Docker comes with three built-in networks:

To see these networks:

docker network ls

Example output:

$ docker network ls
NETWORK ID     NAME      DRIVER    SCOPE
cf10938f5edf   bridge    bridge    local
d4f46412e7e9   host      host      local
b5b0ab8c1665   none      null      local

Inspecting the Bridge Network

To view details such as subnet configuration and gateway:

docker network inspect bridge

Key fields:

[
  {
    "Name": "bridge",
    "Driver": "bridge",
    "IPAM": {
      "Config": [
        {
          "Subnet": "172.17.0.0/16",
          "Gateway": "172.17.0.1"
        }
      ]
    },
    "Options": {
      "com.docker.network.bridge.default_bridge": "true",
      "com.docker.network.bridge.enable_icc": "true",
      "com.docker.network.bridge.enable_ip_masquerade": "true",
      "com.docker.network.bridge.host_binding_ipv4": "0.0.0.0"
    }
  }
]

Running Containers on the Default Bridge

When you start a container without specifying a network, it’s attached to bridge:

docker run -itd --name first centos:7

Inspect its network settings:

docker inspect first --format '{{json .NetworkSettings}}' | jq

{
  "Gateway": "172.17.0.1",
  "IPAddress": "172.17.0.2",
  "IPPrefixLen": 16,
  "MacAddress": "02:42:ac:11:00:02",
  "Networks": {
    "bridge": {
      "IPAddress": "172.17.0.2",
      "Gateway": "172.17.0.1",
      "MacAddress": "02:42:ac:11:00:02"
    }
  }
}

Create a second container:

docker run -itd --name second centos:7

On the default bridge, embedded DNS is not enabled. Attempting to ping by container name fails:

docker exec first ping -c 2 second
# ping: second: Name or service not known

Creating a User-Defined Bridge Network

User-defined bridge networks include built-in DNS and automatic name resolution. Create one with a custom subnet:

docker network create \
  --driver bridge \
  --subnet 192.168.10.0/24 \
  kodekloudnet

Verify its presence:

docker network ls

$ docker network ls
NETWORK ID     NAME           DRIVER    SCOPE
cf10938f5edf   bridge         bridge    local
d4f46412e7e9   host           host      local
f22c791ef1ad   kodekloudnet   bridge    local
b5b0ab8c1665   none           null      local

Running Containers on the Custom Network

Launch two containers on kodekloudnet:

docker run -itd --name customfirst --net kodekloudnet centos:7
docker run -itd --name customsecond --net kodekloudnet centos:7

They now receive IPs within 192.168.10.0/24, and DNS-based name resolution works:

docker exec customfirst ping -c 4 customsecond

PING customsecond (192.168.10.3): 56 data bytes
64 bytes from customsecond.kodekloudnet (192.168.10.3): icmp_seq=1 ttl=64 time=0.07 ms
...

Connecting an Existing Container to a Network

By default, containers attach only to the default bridge. To connect first to kodekloudnet:

docker network connect kodekloudnet first

Verify both network endpoints:

docker inspect first \
  --format '{{json .NetworkSettings.Networks}}' | jq

{
  "bridge": {
    "IPAddress": "172.17.0.2"
  },
  "kodekloudnet": {
    "IPAddress": "192.168.10.4"
  }
}

Now ping first from customfirst:

docker exec customfirst ping -c 2 first

Disconnecting a Container from a Network

To detach a container:

docker network disconnect kodekloudnet first

After disconnecting, customfirst will no longer reach first on that network.

Removing Networks

Docker prevents removing networks with active endpoints. To delete kodekloudnet:

1. Stop and remove containers:

   docker container stop $(docker ps -q)
   docker container rm $(docker ps -aq)
   

2. Remove the network:

   docker network rm kodekloudnet
   

You can also prune all unused user-defined networks:

docker network prune

Links and References

• Docker Networking Overview
• Docker Network Commands
• Kubernetes Networking