What is an address on Ethereum?

#Twenty bytes with a 0x in front

An Ethereum address is just a number, 20 bytes long, that the ecosystem writes in hexadecimal with a 0x prefix. That gives the familiar 42-character string.

0x2170Ed0880ac9A755fd29B2688956BD959F933F8
   |__|                                    2 chars: the 0x prefix
       |____________________________________|  40 hex chars = 20 bytes

It is the name of an account: where value is held, and where transactions are sent. It is meant to be shared. You give someone your address so they can pay you, exactly as you would a bank account number, and knowing it lets anyone look up your balance and history but never move your funds. Only the private key can do that, and the address is not the key.

#Two kinds of account behind an address

Every address is one of two types, and they look identical from the outside.

• An externally owned account (EOA) is a person's account. It is controlled by a private key, and a human (or their software) authorises its actions by signing with that key. It has a balance but no code.
• A contract account is a smart contract. It has a balance and code, and it is controlled by that code rather than a key. It cannot start a transaction on its own; it only runs when something calls it.

You often cannot tell which is which just by looking, which matters for security: sending tokens to a contract that was not written to receive them can lock them forever, and a "wallet" that is actually a contract signs differently from one backed by a key.

#Where an address comes from

For a person's account, the address is derived from their public key. The key is hashed with keccak-256, and the last 20 bytes of that hash become the address. The chain of secrets runs one way only: private key produces public key produces address, and none of those steps can be reversed.

address = last_20_bytes( keccak256( public_key ) )
// private key  -->  public key  -->  address
// each arrow is one-way: you cannot walk it backwards

A contract's address is computed differently, because a contract has no key. When someone deploys a contract, its address is derived from the deployer's address and a counter (the deployer's nonce), so it is fixed and predictable the moment it is created. A variant, CREATE2, lets a deployer choose an address in advance from a chosen salt, which is how some wallets and factories know a contract's address before it exists.

#Why some letters are capitalised

You may have noticed addresses mix upper and lower case. That is not decoration. Hex only needs the digits 0-9 and a-f, so case would normally be meaningless. Ethereum reuses it as a checksum (defined in EIP-55): the capitalisation of each letter is set by the hash of the address itself. A wallet can therefore check that an address is internally consistent and warn you if a single character was mistyped, catching the kind of copy-paste error that would otherwise send funds into a void. If a tool shows an address in all lower case it simply has not applied the checksum; the address is still valid.