Notice: Skynet continues to be developed - information will be added as it becomes available and new features are implemented.
Skynet was announced in February 2020 as a revolutionary step in the Internet and how content will be shared. Skynet is essentially a basic content distribution network (CDN) which is able to serve content on the Sia network to users publicly. Prior to Skynet, there was no way to easily share files stored on Sia without either sharing your Sia installation and seed, or without building your own complex custom front-end website or service that used Sia as a back-end storage provider. With Skynet, anyone can put public content and data on the Sia network, and provide a link for others to access the content.
Skynet has three major components: server-type modules called Portals, links to content called Skylinks, and the traditional Sia storage network.
A Portal is a piece of software which anybody can run that allows access to all Skynet content on the Sia network. A Portal is essentially just a proxy web host/server running on a machine with a Sia client, which accesses the content on the Sia network and serves it through the Portal. By default, any Portal can access any Skynet upload or Skylink on the Sia network, though a Portal operator could implement controls on who can use their Portal, what content can be accessed, or any other number of customizations. There are several Portals currently in operation, and you do not need to run a Portal yourself in order to upload or download Skynet content. However, if you want to ensure that your Skynet content is always available to others, running your own Portal is the best way to do so.
A Skylink is a link to a file uploaded to Skynet. After a Sia renter uploads content to Skynet, a public Skylink is created, and the content can be accessed through a Portal with this link. Skylinks are similar to magnet links for torrents - they're a type of pointer which can be used to find Skynet content stored on Sia.
Skylinks (and therefore, files on Skynet) are "pinned" by users wishing to keep the content online, starting with the original uploader - therefore, pinning content means paying the storage fees for the content. Anyone can continue to pin a Skylink, effectively keeping it online and accessible forever. Skylinks normally stay pinned unless the uploader unpins them, or the contract to store the file is allowed to expire in the same way that a regular Sia renter contract would (i.e. the pinning user does not open Sia within the renewal window for the contract). If multiple people pin the same file, it will stay online unless they all unpin it or let it expire.
The Sia Storage Network
Skynet still uses the main Sia storage product and network to store files. Sia renters can choose to designate uploads as Skynet items, which uploads the content to Sia and creates a Skylink to the content. Because Skynet data resides on Sia, Sia hosts may end up storing Skynet data as a result. From a hosting perspective, this is important to know because most Skynet data is stored unencrypted on Sia hosts, and a full file is stored on each host. As such, as a host you may be liable for any Skynet data you store if it is of an illegal nature. If you are a Sia host or considering hosting, please review the Risks of Hosting on Sia, specifically the risks related to Skynet.
Unlike normal Sia renter storage, Skynet uploads files in one piece with 10x redundancy by default. Therefore, costs to store files on Skynet may be higher than costs as a normal renter due to the extra redundancy. The redundancy level for files can be set manually, but setting it too low may result in delays or slow download speeds when accessing a file. Also, normal Sia renter files can't be converted directly to Skynet files due to this limitation, though this is slated to change in Sia v1.5.0.
Skynet can be used in many ways - through your local Sia client, or through a public Portal. Many public Portals allow you to upload and download Skynet data for free. Locally, Skynet can be used to upload new data to Skynet, and to re-pin existing content which you want to keep online. However, currently Skynet data can only be accessed (downloaded) through a Portal - there is no way to download data from Skynet using your own Sia-UI without also running a Portal configuration of one type or another.
Please see our Guide to Using Sia Skynet for detailed information on how to upload and download data to/from Skynet, as well as how to become a Portal operator. Also see our Skynet FAQs for answers to Skynet questions and help with troubleshooting Skynet issues.
There are a number of public Skynet web Portals in existence which allow you to upload and download Skynet data at no cost to you, and anonymously. "Premium" portals may also exist which allow access to Skynet, or to the Portal operator's specific content, under some sort of monetization scheme.
A list of known Portals and Portal operators can be found on the Skynet Portals page.