Revision · The Network & Nodes

Part 5 is the plumbing beneath consensus: a leaderless gossip network where every node verifies everything for itself, so nobody has to trust anybody. Its throughline is that the rules live in the software each person runs, not in any server or company.

What this part covered

• Node types on a verify-vs-trust dial — full and pruned nodes re-derive every consensus rule and accept a block only if it passes their own checks; SPV clients verify only proof of work plus a Merkle proof and trust that the rules inside were obeyed, so they can’t detect a block that prints extra coins.
• Pruning keeps the conclusion, not the trail — a pruned node validates every block from genesis but discards raw block data afterward, because the state you need is the small UTXO set, not the hundreds of GB of history.
• A flat, permissionless P2P mesh — every node is client and server, finds peers via DNS seeds and addr gossip, and leans on the few outbound links it chose while treating inbound peers as untrusted.
• Announce-then-request gossip — the inv → getdata → tx/block split ships a 32-byte hash first and the heavy data only to peers who lack it, and every node validates before relaying, making the network self-cleaning.
• Fast propagation is a security property — slow block relay widens the window for competing blocks and orphans, so compact blocks (BIP 152) rebuild a block from the receiver’s own mempool to shrink latency and level the field between large and small miners.
• There is no global mempool — each node keeps its own set of consensus-valid, policy-filtered unconfirmed transactions ordered by fee rate, so timing, policy, and RBF replacements guarantee mempools diverge and “it’s in the mempool” is a weak guarantee.
• Initial block download — a new node trusts only the hard-coded genesis block, uses headers-first sync to pick the most-work chain cheaply before pulling bodies in parallel, and treats assumevalid/assumeutxo as self-correcting speed knobs that never skip the supply checks.
• Network attacks — proof of work secures which chain is true but not which peers you talk to, so cheap fake IPs enable eclipse, Sybil, and partition/BGP attacks, countered by diverse outbound peers, anchor connections, and running your own node.

The takeaway

The network layer’s whole job is to make sure every honest node can hear about every transaction and block, so independent verification always has something to verify. Consensus answers “which chain is true”; gossip and connectivity defenses make sure that answer can actually reach you. Part 6 turns to the economics — why the participants with the most power to corrupt the ledger are the ones with the most to lose.