The node

A Bitcoin node is a computer that runs the Bitcoin software and connects to other computers that do the same. It checks each transaction and block against the rules. It keeps its own copy of the record. Running your own node means you check the rules yourself instead of trusting someone else's answer.

Each node is one part of the network. Every node runs the same rules, so no node has to take another node's word for anything. For a shorter definition, see what a node is.

What does a node do?

A node does three things. It checks transactions against the rules, it passes information on to its peers, and it stores every confirmed transaction in its copy of the blockchain.

1. Follow the rules

Before a node passes on any transaction, it checks three things. Do the coins exist, and have they not been spent already? Is the signature valid? Do the outputs add up to no more than the inputs?

Take a payment that tries to send more coins than the sender owns. The node rejects it, so it does not pass it on. The bad payment goes no further than the first node that finds the problem.

2. Share information

Sharing is the node's main job. It passes two kinds of data to its peers. The first is new transactions that are not confirmed yet. The second is blocks of confirmed transactions. A block holds many transactions, so it travels across the network as one piece. To learn more, read about transactions and blocks.

3. Keep a copy of the blockchain

Each node keeps the blockchain, the list of every block since the first one in January 2009. When a new block passes its checks, the node adds it to the end of that list. Making those new blocks out of fresh transactions is the job of mining. If a node falls behind, it asks its peers for the blocks it missed, and so it catches up.

Why run your own node?

Your node checks every transaction and block itself. You do not have to trust anyone else's answers about the record. No one gives your node orders. It makes its own decisions, using the same rules as every other node, so all honest nodes reach the same answers.

Many wallets ask a server for your balance and confirmations. If that server is not yours, you trust its answers. A node you run removes that step, because your own copy of the software does the checking.

How does a node connect to others?

When you start a node, it connects to other nodes. By default, Bitcoin Core makes eight outgoing full-relay connections, which pass on both blocks and transactions. It also makes two block-relay-only connections, which exchange only blocks. Other nodes can connect to you too. Bitcoin Core 32.0 and later accept up to 200 connections in total by default, and earlier releases allowed 125. The limits are set in net.h, in the source code.

Mainnet nodes listen on TCP port 8333. You may need to open that port on your router so that other nodes can reach you. Your node still works without it, but it helps the network when others can connect in.

Do I have to run a node to use bitcoin?

No. You can send and receive bitcoin without running a node. Your wallet builds the transaction and sends it to one or more nodes. Those nodes pass it on, and the payment usually reaches the whole network.

The trade-off is trust. When you use a node you do not run, you trust its answers about balances and confirmations. When you run your own node, your own copy of the software checks those answers.

Does Explore21 run a node?

Yes. Every page on our explorer reads from our own Bitcoin node. You can open the latest blocks to see what it has checked, or go back to an older one, such as block 100,000.

What should you read next?

Next, read mining. It explains how new blocks are made, and why the network needs miners as well as nodes to keep the record moving.