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...