What is a layer 2 blockchain?
A layer 2 is a blockchain network that processes transactions on top of a main chain such as Ethereum, then posts compressed results back to it for security. Rollups such as Arbitrum, Optimism and Base are examples.
Also called: L2, layer 2 blockchain
Why it matters
Fees on Ethereum itself can make small or frequent actions too expensive; a layer 2 brings them down to a level that suits mass-market apps. Arbitrum, Optimism, Base and Polygon all run the same Solidity code and EVM tools, so the choice rests on withdrawals, who runs the sequencer and where your users already are.
1How rollups workTransactions are batched off the main chain, and the batch data or a proof is posted to Ethereum.
2Optimistic vs ZKOptimistic rollups assume batches are valid unless challenged; ZK rollups post a cryptographic proof with each batch.
3Not the network layerIn networking, layer 2 is the data link layer of the OSI model, where switches work; it is unrelated to blockchains.
ExampleA logistics platform anchors fingerprints of many delivery acts and cargo photos on a blockchain every day. On Ethereum itself, fees for that many records would be a noticeable cost, so the smart contract runs on a layer 2. Users never see the chain, and any record can be checked at any time.
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Related terms
BlockchainA shared ledger copied across many computers, where records are added in blocks and cannot be quietly changed.Smart contractA program stored on a blockchain that runs automatically when its conditions are met.DAppAn app whose core logic runs in smart contracts on a blockchain, used through a crypto wallet.
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