The Ethereum main network is public, meaning anyone is free to join and utilize the network. There are no permissions involved: not only can users send and receive transactions, they can also take part in a consensus, as long as they have the appropriate hardware to mine blocks. All parties in the network are mutually distrustful, but are incentivized to remain honest by the mechanisms involved in the PoW consensus. This is an example of a public, permissionless network. These networks offer high resistance to censorship and good data persistence, but are less performant due to the decentralized nature of consensus.

The idea of permission, when applied to blockchains, could take one of several forms: it could be explicit, as in the case of an access control list, or implicit, as in a requirement placed on users to enable them to join a network. An example of a public, permissioned blockchain, could take the form of a public proof of stake network, in which permission to participate in the network as a validator is granted in exchange for a deposit or stake.

Of more concern for this chapter is the idea of networks run by companies, either for their own internal use, or as a shared resource used by a group of companies wanting to transact. Such networks are private: they are not open to the general public, but only to those companies authorized to join. These are sometimes also referred to as consortium blockchains, and as such, networks often also impose different types of permissions for different participants. For example, different members of the network may have read, write, or access permissions, while another subset of members may take on the job of validation.

Such private networks can take advantage of permissioning to be more performant than their public and permissionless counterparts. If a set of nodes can be trusted as validators, then consensus can be reached more quickly. There is, however, a tradeoff: a set of permissioned validators must be trusted to correctly and honestly validate blocks. In the world of business, where time is money, such a tradeoff can often be worth making.