A full node is a program, most often Bitcoin Core, that downloads every block since 2009 and checks every rule on every transaction itself. It does not ask anyone whether a block is valid: it finds out.
- Validates: signatures, no double spend, no coins created out of nothing, block size, proof of work, difficulty.
- Stores: the chain, several hundred gigabytes, and the current set of unspent outputs.
- Relays: passes valid transactions and blocks to its peers, and refuses invalid ones.
- Serves: answers your own wallet, so it never has to ask a stranger's server.
Initial sync
The first start replays the whole history, block by block, to rebuild the current state from scratch. It takes from several hours to a few days depending on the machine. At the end, the node knows the exact balance of every output without having trusted anyone.
▶ SEE IT IN THE MACHINEAt NODES, the block passes through ten rings, and each one checks it against its own full copy before passing it on.