An Underpass chat over the internet takes place on a specific port. See Basic concepts for more information about internet ports. Both devices use the same port for the chat. Underpass automatically selects a default port number, and you can just keep the default setting if you only chat with one other device over the internet. However, if you chat with multiple devices over the internet, you may want to select a different port number for each chat. Different port numbers will allow multiple simultaneous internet chats.
If you want to accept chats that are started by another device, then you need to turn on "Listen for partner".
If another device is already listening for a connection, and you want to start a chat immediately, you need to enter the other device's internet address.
Once you've entered an address, you can start a chat from the chat window.
You can always stop the chat immediately from the chat window, regardless of which device started the chat.
The internet address in the settings will be remembered, so after you stop the chat you'll be able to start again easily without having to enter the address every time.
If you want either device to be able to start a chat, then each device should have the other device's internet address, and each device should automatically listen for a partner. For example with two iPhones:
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