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A private Blockchain Network (also known as restricted blockchain) requires an invitation and must be validated by either the network starter or by a set of rules put in place by the network starter. Businesses who set up a private blockchain, will generally set up a permissioned network. This places restrictions on who is allowed to participate in the network, and only in certain transactions. Participants need to obtain an invitation or permission to join.
Source: P. Jayachandran, The Difference between public and private blockchain, 31 May 2017