Nodes are a fundamental element of the Bitcoin network architecture, as they perform various crucial functions:

Therefore, any device running a piece of Bitcoin software, called a Bitcoin node (often using Bitcoin Core), contributes to the decentralization of the network.

Each node holds a copy of the blockchain, which permits transaction verification and prevents any fraud attempt. The decentralized nature of the network gives Bitcoin exceptional resilience and robustness. In fact, to stop the Bitcoin protocol, all nodes around the world would have to be shut down. As of September 2023 there were approximately 45,000 nodes distributed across the globe.

Nodes are capable of verifying the validity of blocks and transactions because they follow the rules of the Bitcoin consensus. These rules establish the Bitcoin's monetary policy, such as the mining reward amount (which we will discuss in more detail in the next section) and the amount of bitcoin in circulation. In a way, nodes act as the network's legal system since they enforce Bitcoin's rules, keeping the network neutral. Consensus rules hardly vary, if at all, because to make changes, the approval of all nodes is required.

Governance within the protocol is beyond the scope of this basic course, but it's important to note that each user running a Bitcoin node can decide which rules to follow. A user may choose to adhere to different rules (i.e., make modifications to the code), but if these changes invalidate the current consensus rules, that node will no longer be part of the Bitcoin network. Consequently, major modifications are rare and require significant coordination among thousands of participants with diverse ideologies and interests, which forces them to provide updates that are considered 'better' by all Bitcoin users.

There are several options available when you want to install your own node, with different maintenance costs. You can simply run the Bitcoin Core software on your computer, but it will require a significant amount of storage space, as the blockchain is about ~500GB. To overcome this constraint, you can choose to only keep the last N blocks in the memory by creating a "pruned node". For this second solution, the cost is negligible because the node is only active when you need it.

A second option is to use a dedicated piece of hardware for this purpose, such as a Raspberry Pi 4 with a sufficiently large SSD (about ~2TB). This other option is more expensive if you have to buy the hardware, but it represents a little less than €10.00 per year in terms of electricity consumption. From a bandwidth perspective, considering 1 block of 1MB every 10 minutes, it corresponds approximately to 5GB per month.

The affordable cost and accessibility of a Bitcoin node in terms of hardware resources, storage, and bandwidth is a very important characteristic, as it facilitates the decentralization of the network.

Indeed, everyone has a good reason to run a node! The costs and efforts are minimal compared to the benefit obtained. You just have to embark on the adventure and join thousands of other bitcoiners to form the Bitcoin network all together.

On the contrary, if the blocks were 100 times heavier, we could certainly make 100 times more transactions every 10 minutes, but running a Bitcoin node would require a 50TB hard disk, a bandwidth of over 500GB/month, and a piece of hardware capable of validating hundreds of thousands of transactions in less than 10 minutes. In this hypothetical situation with 100 times larger blocks, running a Bitcoin node would not be accessible to the average person, which would compromise both the decentralization of the protocol and the immutability of transactions and consensus rules.

Thus, the protocol constraints have been designed to enable as many people as possible to run their own Bitcoin nodes. In fact, the year 2017 was marked by an intense controversy known as the "block size war". This conflict pitted those who wanted to modify Bitcoin by increasing the block size to enhance transaction capacity (miners, exchange platforms, and institutions) against those who sought to preserve the independence and power of users (nodes and users). In the end, the second party triumphed.

Following this victory, the nodes activated an update called SegWit, paving the way for the implementation of the Lightning Network, an instant Bitcoin payment network built as a second layer of the Bitcoin blockchain. This situation demonstrates that users, through their nodes, hold real power within Bitcoin, allowing them to stand up to large institutions in times of disagreement.