You can refer to your local machine in many ways, in different contexts, locally within the same machine or within a private network.

Within the context of our local machine, we can use the 127.0.0.0/8 loopback addresses. Anything sent to the loopback address is sent back to the sender; therefore, we can use 127.0.0.1 to refer to our own machine.

If your computer is part of a private network, your computer will be assigned an IP on this network. These private IP addresses have a limited range, as defined in RFC 1918:

• 10.0.0.0 - 10.255.255.255 (10/8 prefix)
• 172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
• 192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

0.0.0.0 is a special address, which includes both your local loopback addresses and the IP address of your private network. For instance, if your private IP address is 10.194.33.8, anything sent to 127.0.0.1 and 10.194.33.8 will be available for any services which are listening to 0.0.0.0.

Therefore, when we bind 0.0.0.0:9200 to the container’s port, 9200, we are forwarding any request coming into our local machine on port 9200 to the container.

This means that when we run our E2E tests, whenever our backend API is sending a request to localhost:9200, that request is forwarded inside the container via its 9200 port.

You can see all port mappings by using the docker port command:

$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
a415f4b646e3 docker.elastic.co/elasticsearch/elasticsearch-oss:6.2.4 "/usr/local/bin/dock…" 2 hours ago Up 2 hours 0.0.0.0:9200->9200/tcp, 0.0.0.0:9300->9300/tcp elasticsearch
$ docker port a415f4b646e3
9300/tcp -> 0.0.0.0:9300
9200/tcp -> 0.0.0.0:9200