Intro to content caching for Apple devices
Content caching is a service in macOS that speeds up downloading of software distributed by Apple and data that users store in iCloud by saving content that local Apple devices have already downloaded. The saved content is stored in a content cache on a Mac, and is available for other devices to retrieve without going out over the Internet. You can find the current list of supported content types in the Apple Support article Content types supported by the caching service.
You use content caching on networks consisting of a NAT environment for the content cache and all devices, or on networks consisting of publicly routable IP addresses. Apple devices automatically contact a nearby content cache without any configuration.
Important: For best results, deploy content caching on a Mac that has a single wired Ethernet connection as its only connection to the network. Content caching can use a Wi-Fi connection instead of Ethernet, but performance might be affected.
How content caching works
After you enable content caching on a Mac, it keeps a copy of all content that local networked devices (called clients) download.
For example, when the first client on your network downloads a macOS update, the content cache keeps a copy of the update. When the next client on the network connects to the App Store to download the update, the update is copied from the content cache rather than from the App Store
Because the local network is normally much faster than the Internet, the second client (and all subsequent clients) download updates much faster.
The following figure illustrates a single private subnet with a single content cache: