A permissioned blockchain is a type of blockchain in which chain read and/or write access is limited to a known set of validators. Permissioned blockchains have an access-control layer built into the node software, centrally defining the rights that a node has. Since nodes are generally trusted institutions, there is signficantly less risk of a 51% attack. Further, software updates are more efficient to coordinate, since node operators are generally subject to a uniform set of established governance procedures. This approach is in contrast to permissionless blockchains such as Bitcoin or Ethereum, where any entity can run a node and participate in consensus. Ripple is a notable example of a permissioned blockchain, with validation work performed by nodes owned by the Ripple company.