21 BTCFull node
A full node is a computer running Bitcoin software that checks every block and transaction against the rules by itself. With your own node you don't have to trust anyone about your balance.
Short definition
Software that downloads and checks every block and enforces the rules for you.
What a node does
A node downloads the whole Blockchain from the Genesis block on, verifies every signature and rule, and keeps the UTXO set. It relays valid transactions and blocks to its peers and rejects invalid ones. A wallet connected to your own node learns your balance without revealing your addresses to a third-party server.
A pruned node verifies everything too but deletes old block data, so it fits in a few gigabytes. Crawlers can only count “listening” nodes that accept incoming connections, so the real number of nodes is higher than the reachable count.
Software
Most nodes run Bitcoin Core; a growing minority run Bitcoin Knots, a Core derivative with stricter relay policy. Plug-and-play boxes (Umbrel, Start9, RaspiBlitz and others) bundle a node with apps.
See also
- Bitcoin CoreBitcoin Core is the main software that runs the Bitcoin network, the direct descendant of Satoshi's original program. It is free and open source, maintained by volunteers and grant-funded developers.
- Bitcoin KnotsBitcoin Knots is an alternative node program based on Bitcoin Core and maintained by Luke Dashjr. It follows the same consensus rules but filters more non-payment data by default.
- Pruned nodeA full node that deletes old blocks after checking them, to save disk space.
- Electrum serverAn indexer on top of a node that wallets like Sparrow connect to.
- UTXO (unspent transaction output)Bitcoin has no account balances. Your wallet holds separate “coins” called UTXOs, like notes in a purse; a payment spends some of them and creates new ones, including change back to you.
References
- bitcoin.org: running a full node bitcoin.org
- btcnodes.io (reachable nodes) btcnodes.io