Chapter 7 - 0 of 10 quizzes passed
Nodes: The Rule Checkers
Nodes give Bitcoin users power by independently rejecting invalid transactions and blocks.
- Distinguish a full node, a pruned node, and a light client
- Explain what a node validates and what it cannot do
Who runs a node
A node can be run by an individual, a business, an exchange, a miner, or anyone who wants to verify Bitcoin directly. A full node downloads and validates the chain and keeps the UTXO set. A pruned node validates the same rules but discards old block data. A light client trusts a server for some data instead of validating everything itself.
What nodes validate
A full node checks every transaction and every block against the same rules, by itself, before accepting it. The Network Lab simulates a simplified version (P2WPKH signatures only).
Every transaction
- Well formedAt least one input and one output, and within size limits.
- Coins exist and are unspentEvery input points at a real, unspent output. No coin is spent twice.
- The owner approved itSignatures and witness data satisfy each coin’s lock (P2PKH, P2SH, SegWit, Taproot).
- No money from nothingOutputs add up to no more than the inputs (only the coinbase, below, creates new coins); every amount is between 0 and 21 million BTC.
- Timelocks have passedAny locktime or relative delay set on the transaction or its inputs has expired.
- New coins have maturedCoins created by mining can only be spent after 100 more blocks.
Every block
- Enough proof of workThe block header’s hash is below the current target.
- Correct difficultyThe target matches the adjustment rule, recalculated every 2,016 blocks.
- Builds on a known blockIt points at a parent block this node has already accepted.
- Sensible timestampLater than the median of the last 11 blocks, and no more than 2 hours ahead of the node’s clock.
- Contents match the headerThe Merkle root (and the SegWit witness commitment) match the transactions inside.
- Fits in a blockTotal weight is at most 4,000,000 units, and signature operations stay under the limit.
- Miner pays itself correctlyExactly one coinbase, first in the block, claiming no more than the subsidy plus fees. The subsidy halves every 210,000 blocks; this is how the 21 million cap is enforced.
- Every transaction passesEach one passes the transaction checks above, and no two spend the same coin.
Choosing the chain
- Most work wins, among valid chainsThe node follows the valid chain with the most accumulated proof of work, and switches (reorganizes) if a heavier valid chain appears. An invalid chain is ignored however much work it has.
Local policy: what this node relays
Not consensus. Each node sets its own; a transaction that breaks policy can still be valid if a miner includes it.
- Minimum fee rateToo-cheap transactions are not kept or passed on.
- Standard forms onlyUnusual scripts and tiny “dust” outputs are not relayed.
- Replacement rulesA replacement must pay more than the transaction it replaces (RBF).
- Size and chain limitsCaps on transaction size and on long chains of unconfirmed parents and children.
If any consensus check fails, the node rejects the transaction or the whole block, no matter who sent it or how much work it carries.
The power to reject
A node’s power is the power to reject invalid data. If a miner creates a block with fake signatures, already-spent inputs, too much subsidy, or broken scripts, honest nodes ignore it even if the miner spent real energy. This is why miners cannot simply change the supply rule: they would be mining a chain that other nodes do not accept.
What nodes cannot do
Nodes cannot seize other people’s coins or force valid users to pay them. They do not control private keys. Their power is defensive: they decide what their own copy of Bitcoin accepts as valid. A node also does not make you anonymous, and an eclipse attack can cut you off from honest peers. Consensus is not a vote; it is independent validation converging on the same rules.
Network Lab
See it in the simulation
Watch the node reject a transaction with a broken signature.
What you should take away
Full nodes verify the rules for themselves and reject invalid blocks, including blocks from powerful miners. They cannot spend your coins, but they also do not guarantee privacy or immunity from network-level attacks.
Retrieve, then apply
Distinguish a full archival node, a pruned node and a light client.
Check your explanation
Archival and pruned full nodes validate the rules; pruning discards older block data after validation. A light client verifies less and relies on others for some information.
Checkpoint
Sources
Reviewed 2026-10-25 · not financial advice