Software that implements Ethereum spec and talks over p2p network. Ethereum spec is documented in the Yellow Paper as well as several Ethereum Improvement Proposals.

There are many ethereum based networks that are not interoperable.

Pretty pointless chapter, just goes over nodes. The next chapter is about cryptography.

As we saw earlier in the book, Ethereum has two different types of accounts: externally owned accounts (EOAs) and contracts. Ownership of ether by EOAs is established through digital private keys, Ethereum addresses, and digital signatures. The private keys are at the heart of all user interaction with Ethereum. In fact, account addresses are derived directly from private keys: a private key uniquely determines a single Ethereum address, also known as an account

In Ethereum, we use public key cryptography (also known as asymmetric cryptography) to create the public–private key pair we have been talking about in this chapter. They are considered a “pair” because the public key is derived from the private key. Together, they represent an Ethereum account by providing, respectively, a publicly accessible account handle (the address) and private control over access to any ether in the account and over any authentication the account needs when using smart contracts. The private key controls access by being the unique piece of information needed to create digital signatures, which are required to sign transactions to spend any funds in the account. Digital signatures are also used to authenticate owners or users of contracts

Eth uses standard elliptic curve cryptography, with a NIST standard curve. Starting with a private key in the form of a randomly generated number k, we multiply it by a predetermined point on the curve called the generator point G to produce another point somewhere else on the curve, which is the corresponding public key K:

The generator point is specified as part of the secp256k1 standard; it is the same for all implementations of secp256k1, and all keys derived from that curve use the same point G. Because the generator point is always the same for all Ethereum users, a private key k multiplied with G will always result in the same public key K.

Ethereum addresses are hexadecimal numbers, identifiers derived from the last 20 bytes of the Keccak-256 hash of the public key.