how to publish/subscribe with remote roscore?

edit

Current setup: 1. roscore running on my remote server ( a virtual private server ) VPS 2. Machine A and B are connected to internet 3. Machine A publishes a topic "test_topic"

Result: 1. Machine A can subscribe to the topic using rostopic echo /test_topic 2. BUT Machine B CAN NOT subscribe to the topic ( using rostopic echo /test_topic or using python/c++) 3. VPS, where roscore is running, CANNOT subscribe to the topic

However, rotopic list shows "/test_topic"!

So, is it possible to use a roscore running on a remote server and nodes connecting over internet?

Thanks.