Macs and Apple devices now support several ways of sharing displays and their control using mouse, trackpad and keyboard. This article tries to deconfuse them and explain their different usage.

These fall into three groups of features:

• Continuity, introduced recently and confined to Apple products, and built around Macs and devices signed into the same Apple ID, with both Wi-Fi and Bluetooth running;
• VNC, an old set of cross-platform protocols available on most computers and some devices, using any form of network connection, including the Internet to remote systems;
• Firmware features making a Mac’s display or internal storage accessible in a Target mode.

Apple provides an overview of Continuity in this support article, while VNC is explained on Wikipedia.

Continuity

Apple started integrating Macs and devices nearly ten years ago in OS X Yosemite with Handoff between iPhones and Macs. Recent versions of macOS (from Monterey), iOS, iPadOS and others extend these to include sharing of displays and control devices such as mice, trackpads and keyboards. Fundamental requirements include Wi-Fi networking and Bluetooth, used both to negotiate and maintain the connection, and some form of mutual authentication. The latter is normally the requirement for both Macs or devices to be logged in using the same Apple ID, but can also be extended to others on the same network. These services use proprietary protocols and thus aren’t able to connect to computers or devices from other vendors.

The first step in setting up Continuity’s AirPlay is to open AirDrop & Handoff settings in the General section of System Settings, on the Mac whose display you want to view remotely. AirPlay Receiver should be turned on, and you then need to decide who to allow it for, whether just the Current User who’s signed in with the same Apple ID, others on the same network, or everyone (which you should normally avoid). For the latter two categories you should require a password.

Connecting to that display should then be possible by selecting it in the Displays settings of the other Mac.

The AirPlay display then appears as a secondary display, and you can switch between mirror and extend modes and rearrange the displays as you wish.

SharePlay Display is different again, and allows you to share during FaceTime calls; Apple explains that in another support article.

AirPlay and SharePlay share the whole or parts of the display, but don’t allow you to control both Macs using the same mouse, trackpad or keyboard, which is the task of another part of Continuity, Universal Control. To enable that you’ll require the same basic setup, with Wi-Fi, Bluetooth and both Macs signed in with the same Apple ID. Because its release was delayed during the Monterey cycle, it requires macOS 12.4 or later. Its controls are hidden away behind the Advanced… button at the foot of Displays settings.

For most, the easiest way to use Universal Control is to allow the pointer to push through the edge of the display of one Mac to the other; to enable that, set the upper two options on. Switch back to Displays settings with Universal Control active, and you can then fine-tune the settings, arranging the displays as you want. Apple provides a full account in this support note.

For most, Continuity works reliably, although a few report persistent problems that may be the result of unreliable Wi-Fi or Bluetooth connections, interactions with third-party software or peripherals. Stripping the two systems down to a bare minimum to establish AirPlay or Universal Control is usually a good approach to troubleshooting.

VNC

Virtual Network Computing was developed over a quarter of a century ago to enable computers to connect to and share displays with others, and to control them, over a network. Its underlying RFB protocol has been widely implemented on PCs running Windows and Linux, and even Raspberry Pi, and Macs can act as clients or servers to those as well as other Macs and devices. RFB provides a means for a server to supply a stream of data reflecting the changing display contents over a network connection to the client, which then reassembles that into a local image of what’s on the remote display. Local controllers such as mice, trackpads and keyboards can also be used to control the remote server, and there are popular extensions to support features such as file and clipboard exchange. One notable exception is audio, which for largely historic reasons remains unsupported.

macOS comes with VNC server software that needs to be enabled in the Sharing section of General settings. For basic sharing and control features, turn on Screen Sharing there, and set its options up. Connect to that Mac using the Screen Sharing app in /Applications/Utilities. This used to be hidden away in /System/Library/CoreServices/Applications, but in Sonoma has at last been made more accessible. Apple describes how to use this app in this support article.

VNC is an important tool for those who support Macs remotely, whether they’re paying clients or relatives. For those who need more extensive features, Apple Remote Desktop is sold in the App Store, where there are third-party alternatives including Screens 5. The latter is particularly attractive for making most tasks far simpler; for instance, an ordinary user can readily establish a remote connection via the Internet using its tools. ARD and more advanced apps require the server Mac to have Remote Management enabled, rather than Screen Sharing.

Screen sharing and remote management have one other valuable use: they can be used to connect to a local virtual machine running macOS (on Apple silicon Macs), and significantly enhance their features, as explained here.

Establishing VNC connections on a local network, inside a firewall, should be straightforward and perform well. The only complication is Sonoma’s new high performance option, which may not work at all in some circumstances. Over the Internet, though, VNC can be more of a challenge, and I recommend a commercial implementation such as Screens 5 to save time and effort.

Target modes

Depending on model, different Macs can be used as ‘dumb’ servers or Targets, enabling some iMacs to function as a secondary display to another Mac, and all to function as external storage. Two such Target modes are relevant here: Target Display, and Target Disk.

In Target Display mode, an iMac’s display is used as an external display to another compatible Mac. Unfortunately, this capability has been built into disappointingly few models, and there are also strict limits on which versions of macOS can be used. No recent models can be used as either Targets or their connections. Apple explains this in detail in this article.

Target Disk mode is universally supported, although Intel and Apple silicon models implement it quite differently. To put an Intel Mac into Target Disk mode, start it up with the T key held down until the characteristic T icon appears on its display. It should then mount as an external drive on the Mac to which you connect it, using a USB or Thunderbolt cable. Apple’s instructions are here.

Apple silicon Macs are completely different, in that the Target Mac has to be put into Recovery mode, where the Share Disk command in its Utilities menu makes that disk available via a USB or Thunderbolt cable. The other Mac then connects to it as if it were being shared on a network, using SMB. Apple’s detailed instructions are here. Although that might not appear promising, transfer speeds of around 1 GB/s should be achieved for both read and write, over a Thunderbolt cable.

Summary

• Continuity for AirPlay’s shared display and Universal Control’s shared controllers.
• VNC for Screen Sharing and Remote Management, including other platforms and over the Internet.
• Target Disk mode to access internal storage from another Mac.