[Docker] – Understanding Docker Container Networking

Docker creates automatically 3 networks as follows.

$ docker network ls

NETWORK ID          NAME                DRIVER              SCOPE

9efac8172982        bridge              bridge              local

34f8b39ae485        host                host                local

23e44cc8fd8c        none                null                local

The bridge is the default network which is used when a container is created. Then, it means that by default the packets get routed between containers on the host via it (docker0).

Also, we are able to create new networks as the following example.

docker network create -d bridge my_bridge

And, then we are able to make our containers use our networks as the following example

 docker run -d --net=my_bridge --name my_container

Following an image with an example to illustrate this idea.

How to find the Docker containers IP? 

Use the command docker network inspect <network> to get this information.

Example:

$ docker network inspect bridge
[
    {
        "Name": "bridge",
        "Id": "9efac81729828cac8ac66f1f5842e4907af89da7bb716adca0b10068db0e99e3",
        "Created": "2019-10-18T00:45:06.391122672Z",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.17.0.0/16",
                    "Gateway": "172.17.0.1"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            "0dfef3a16d0f0c99abef41085088d4bf3e492af7da1c6acdd5717bd35507831f": {
                "Name": "nginx",
                "EndpointID": "7c0a9a02ff8bfe573e799e05cc47da76085b9e9c2e10d09ecaac95d7c6a4cc8c",
                "MacAddress": "02:42:ac:11:00:03",
                "IPv4Address": "172.17.0.3/16",
                "IPv6Address": ""
            },
            "f69da87bec9f15e8db5de693ace3a0a11fb31768df869feeb3caaaee256dc939": {
                "Name": "my_container",
                "EndpointID": "f59b98043ced8808fccadc58e5eaf179285aca57545dc51ff83da2905d9886f9",
                "MacAddress": "02:42:ac:11:00:02",
                "IPv4Address": "172.17.0.2/16",
                "IPv6Address": ""
            }
        },
        "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",
            "com.docker.network.bridge.name": "docker0",
            "com.docker.network.driver.mtu": "1500"
        },
        "Labels": {}
    }
]

See here the IP for my_container in this network for example. Then, by using it would be possible to ping the my_container from any other container inside of this network.

   “f69da87bec9f15e8db5de693ace3a0a11fb31768df869feeb3caaaee256dc939”: {

                “Name”: “my_container”,

                “EndpointID”: “f59b98043ced8808fccadc58e5eaf179285aca57545dc51ff83da2905d9886f9”,

                “MacAddress”: “02:42:ac:11:00:02”,

                “IPv4Address”: “172.17.0.2/16”,

                “IPv6Address”: “”

            }

NOTE: For a further understanding check the Docker docs.

Following some exercises which can help you learn more about it.

• Docker Networks: https://www.katacoda.com/courses/docker/networking-intro
• Communicating Between Containers: https://www.katacoda.com/courses/docker/5

Also, please feel free to check the tag docker of this website to see all related blog posts.

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