A light node, also called a light client, is a piece of software that takes part in a blockchain network without downloading and checking every block in full. Instead, it stores the chain of block headers, the short summaries at the top of each block, and asks other nodes for small cryptographic proofs whenever it needs to confirm a particular transaction or balance. This lets phones, browsers and small devices interact with a blockchain while using a tiny fraction of the storage, bandwidth and processing power of a full node.
The idea dates back to the original Bitcoin white paper of 2008, which described "simplified payment verification" (SPV). Light clients are now used on most major chains, including Bitcoin and Ethereum, and they play a growing role in cross-chain bridges, where one chain needs to check events on another without running its full software.
Light node in plain terms
Imagine a library that keeps a complete copy of every newspaper ever printed. Checking a story there is easy but needs a huge building. A light node is like a person who keeps only the front-page headlines and dates of each issue. When they want to check a particular story, they ask the library for that one page plus a stamped receipt proving the page really belongs to the issue they already have on file.
The person can be sure the page is genuine without storing the whole archive. What they give up is the ability to check every story in every issue for themselves; they rely on the fact that the headline list is backed by the work of the whole network.
How a light node works
- The light node connects to other nodes and downloads block headers, checking that each header links correctly to the one before and, on proof-of-work chains, that it carries valid work.
- When the user wants to confirm a payment, the light node requests a Merkle proof showing that the transaction is included in a given block.
- It hashes the proof up to the root stored in the block header. If the result matches, the transaction is confirmed as included.
- On account-based chains such as Ethereum, similar proofs can show the current balance or contract storage of an address.
On Ethereum's proof-of-stake chain, light clients follow a rotating "sync committee" of validators introduced in the Altair upgrade of 2021, whose signatures attest to the latest headers. This lets a light client keep up with the chain without tracking every validator. Because proof-of-stake chains have no accumulated work to compare, a new light client usually needs a recent trusted checkpoint to start from, a requirement known as weak subjectivity.
Light nodes differ from wallets that simply query a company's server. Such server-based wallets are convenient but must trust the server completely, whereas a true light client checks headers and proofs itself and only trusts the network's majority.
Light nodes compared with full nodes
| Feature | Light node | Full node |
|---|---|---|
| Data stored | Block headers and requested proofs | Every block and the current state |
| Checks all rules | No; mostly checks headers and inclusion | Yes, every transaction and block |
| Hardware needed | A phone or browser can be enough | Significant disk space and bandwidth |
| Trust in peers | Relies on honest peers and the majority of miners or validators | Independent verification |
Uses
- Mobile wallets. Many mobile wallets use light-client techniques, or connect to a trusted server, because phones cannot hold a full chain.
- Bridges and interoperability. On-chain light clients let one blockchain verify headers and proofs from another, a safer basis for blockchain interoperability than a small group of signers.
- Embedded devices. Point-of-sale terminals and Internet of Things devices can check payments with limited resources.
Security trade-offs
A light node accepts the chain that the majority of block producers have built, but it does not check every transaction against every rule, so it could be fooled into following an invalid chain if most miners or validators misbehaved. It can also be denied information, or shown a misleading view, if it connects only to dishonest peers. Privacy is weaker too: asking peers for proofs about specific addresses can reveal which addresses belong to the user, a problem that newer Bitcoin light-client designs try to reduce with compact block filters. For large sums or business use, running a full node remains the stronger option, and the overall health of a network depends on enough independent full nodes existing for light nodes to rely on. These issues sit within the wider question of how many nodes a network needs to stay decentralised.