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Basic concepts

A chat in Underpass involves two devices: your Mac and another Apple device. The other device can be a Mac, iPhone, iPad, or iPod touch. You can participate in multiple chats simultaneously, with multiple devices, but each of the chats occurs in a separate window.

For two devices to participate in a chat, one device listens for a chat, and the other device starts the chat. This is similar to a phone call. When you turn on your phone, it automatically starts waiting for a call. When another person wants to talk to you, they make a call with their phone, dialing your phone number. Either person can wait for a call, and either person can make a call, but a phone call requires that one person make it and the other receive it. Once you're on a call, either person can hang up. The same ideas apply to Underpass.

Before you can have a chat in Underpass, at least one device needs to know how to contact the other device. Likewise, before you can have a phone conversation, the caller needs to know the receiver's phone number. It's not necessary for the receiver to know the caller's number, but it certainly helps if you want to call them back. Finding an Underpass device is usually simpler on a Local Area Network (LAN) than on the internet.

A wired or wireless network in your home, your office, or a coffee shop is a LAN. Most LANs cover only a single building or an even smaller area within a single building. On a LAN, Underpass can use the Mac's built-in Bonjour automatic service discovery technology to identify and locate another device. Bonjour gives each device on the LAN a unique computer name. You can see your own Mac's computer name in the Sharing pane of System Preferences.

System Preferences, Sharing

When you want to start an Underpass chat on a LAN, you select your partner's computer name in the chat settings.

Bonjour Settings

On the internet, a computer is located by an internet address. There are several different forms of internet address: IPv4 addresses such as, IPv6 addresses such as 2001:db8:ffff:1:201:02ff:fe03:0405, and domain names such as A network service on a computer also requires a port number. A port is analogous to a phone number extension, which you use when many people can be reached at the same phone number. Likewise, a computer has many ports because multiple internet services can run on the same computer. An Underpass chat is one type of network service. Internet port numbers run from 0 to 65535, but the use of port numbers below 1024 requires administrator authorization, so Underpass only works with ports 1024 and above. When you want to start an Underpass chat on the internet, you enter your partner's internet address and port in the chat settings.

Internet Settings

Next: Start a chat

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