AllStar is a digital linking network used to link repeaters, or nodes, over the Internet to other repeaters or nodes.
According to their web site:
The AllStar Link network consists of a number of large (and small) individuals and groups who provide efficient, large-area communications to the Amateur Radio public in their respective local areas. This is done by providing a local VHF or UHF repeater system controlled by a Linux-based computer system running the open-source Asterisk PBX telephone switch platform along with the app_rpt repeater/remote base controller/linking software module (which is included in the distribution of Asterisk) connected to a high speed (broadband, such as Cable Modem or DSL) Internet connection.
The computer system running Linux/Asterisk PBX coupled with the app_rpt module makes a repeater/remote base controller capable of controlling many repeaters and/or remote bases per computer system. It provides linking of these repeater and remote base “nodes”, with “nodes” on other systems of similar construction anywhere in the world, over the Internet via its IAX2 Voice Over IP protocol.
AllStar Link is an organization devoted to the proliferation of this technology, and to organize its public use. Certainly, anyone can have a “private” system using this technology, and they would have no need for AllStar Link affiliation, but there needs to be a single, central point of organization for public use of this technology, and that's what AllStar Link provides.
The technology has the unique characteristic that repeaters and remote base nodes are completely separate from each other, unlike any other repeater/remote-base controllers. That means that just because a remote-base is at the same site or even on the same computer system as a repeater, they are not tied together in any way. They are implemented as completely separate nodes, usable separately.
Unlike other Radio-centric VOIP technologies, such as Echolink or IRLP, etc, Allstar and the app_rpt/Asterisk technology have been specifically designed to be part of, and to link together parts of the very infrastructure of the radio systems that it implements, as opposed to be an end-to-end protocol like others.
All systems (nodes) are either repeater controllers or remote-base controllers. They connect directly with the radio hardware (thus replacing/outdating) current controllers on a system that is already up and operating. Just simply as a repeater controller, the amount of functionality and flexibility is very impressive, and when you also consider its remote base, linking (full-duplex) and VOIP (for autopatch, remote control, etc) capabilities, it's amazing.
Allstar Link is an attempt to take this technology and make it available and applicable to as many Amateur Radio operators as possible, via their local repeater systems.
Repeaters with AllStar capability can be noted on RepeaterBook. The node number can be assigned to the repeater, which is displayed on the repeater's details page. stats.allstarlink.org is scraped in real-time to display the current status of the node.