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port

Usage:  docker port CONTAINER [PRIVATE_PORT[/PROTO]]

List port mappings or a specific mapping for the container

Options:
      --help   Print usage

You can find out all the ports mapped by not specifying a PRIVATE_PORT, or just a specific mapping:

$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS                                            NAMES
b650456536c7        busybox:latest      top                 54 minutes ago      Up 54 minutes       0.0.0.0:1234->9876/tcp, 0.0.0.0:4321->7890/tcp   test
$ docker port test
7890/tcp -> 0.0.0.0:4321
9876/tcp -> 0.0.0.0:1234
$ docker port test 7890/tcp
0.0.0.0:4321
$ docker port test 7890/udp
2014/06/24 11:53:36 Error: No public port '7890/udp' published for test
$ docker port test 7890
0.0.0.0:4321