Layer 2 (L2): What It Is and How It Works

A layer 2 is a network built on top of a base blockchain (layer 1) that processes transactions off the main chain and posts data or proofs back to it. It increases throughput and lowers fees while relying on the base chain for security, and is the main way Ethereum scales today.

By COINOTAG Research DeskLast updated September 30, 2026Sources

How layer 2 networks work

A layer 1 like Ethereum keeps every node verifying every transaction, which is secure but limits capacity and makes gas fees rise when demand is high. A layer 2 moves execution elsewhere and uses the layer 1 as a court of final appeal.

On today's main type, the rollup:

Users send transactions to the layer 2, where a sequencer orders and executes them quickly.
The rollup compresses many transactions into a batch and posts the data to Ethereum.
Ethereum contracts check that the new state is correct, either through a challenge window or a validity proof.
Users can withdraw back to Ethereum using the data on layer 1, even if the layer 2 operator disappears, provided the system is fully decentralized.

The Dencun upgrade on 13 March 2024 introduced blobs (EIP-4844), a cheaper place for rollups to post data, which cut the cost of using many layer 2 networks.

The motivation is the scalability trilemma: it is hard for one blockchain to maximise decentralisation, security and scalability at the same time. Raising a base layer's capacity directly can make nodes more expensive to run and weaken decentralisation. Layer 2s "borrow" security from the base chain and solve scalability on top of it.

Types of layer 2

Type	How it proves correctness	Examples	Notes
Optimistic rollup	Assumes batches are valid; anyone can submit a fraud proof during a challenge period	Arbitrum, Optimism, Base	Withdrawals to layer 1 take days unless a bridge fronts the funds
ZK rollup	Posts a cryptographic validity proof with each batch (zero-knowledge proofs)	zkSync, Starknet, Linea	Faster finality on layer 1; complex to build
Validium	Like a ZK rollup but keeps data off Ethereum	Various	Cheaper, but relies on outside data availability
State channels	Two parties trade off-chain and settle the net result	Bitcoin's Lightning Network	Suited to payments

Sidechains such as Polygon's original PoS chain have their own validators and security, so they are not strictly layer 2. Networks built on top of a layer 2 are sometimes called layer 3.

On Arbitrum and Optimism the challenge period for withdrawals to Ethereum is roughly seven days. Users often shorten the wait with third-party "fast bridges", which add their own trust assumptions.

A short history of layer 2

2016: Joseph Poon and Thaddeus Dryja published the Lightning Network paper, proposing instant payment channels for Bitcoin.
2017: Joseph Poon and Vitalik Buterin published the Plasma paper, proposing child chains for Ethereum. Data-availability problems kept Plasma from wide adoption, but it paved the way for rollups.
2018–2019: rollup designs, which keep transaction data on Ethereum but move execution off-chain, took over as the main approach.
31 August 2021: Offchain Labs opened Arbitrum One to the public; Optimism's mainnet opened to everyone later the same year.
March 2023: Matter Labs opened zkSync Era, one of the first EVM-compatible ZK rollups on mainnet.
2023: Arbitrum and Optimism distributed governance tokens through airdrops.
August 2023: Coinbase opened Base, built on the OP Stack, to the public.
13 March 2024: the Dencun upgrade (EIP-4844) added blob space for rollup data and cut layer-2 fees sharply.

Layer 2s now cluster into ecosystems around shared software stacks: chains built on the OP Stack form the "Superchain", and chains using Arbitrum technology form Arbitrum Orbit. Separate data-availability layers such as Celestia are part of the wider "modular blockchain" approach.

Key terms

Sequencer: the operator that orders layer-2 transactions and packages them into batches.
Bridge: the contracts that move assets between layer 1 and layer 2, or between layer 2s.
Data availability (DA): publishing layer-2 transaction data so anyone can independently recompute the state.
Withdrawal period: about seven days for optimistic rollups; much shorter for ZK rollups once a proof is verified.
Native vs bridged tokens: the same asset can exist on a layer 2 as a natively issued token or as a bridged copy of a layer-1 token; they are not always interchangeable.

In practice: Ethereum mainnet produces a block every 12 seconds, and a DeFi transaction can be a noticeable cost in busy periods. The same transaction on a layer 2 usually confirms faster and costs a small fraction of the mainnet fee. A user typically bridges funds once and then does repeated swaps, lending and NFT trades on the layer 2, which is why most frequent, small-value activity has moved there.

Trade-offs and risks

Centralized sequencers: most rollups still rely on one operator to order transactions, which could censor or go offline.
Upgrade keys: many contracts can be upgraded by a small group; independent trackers such as L2BEAT rate rollups by "stages" of decentralization.
Proof systems: some rollups have not yet fully enabled fraud or validity proofs.
Bridging risk: moving assets between chains through third-party bridges adds contract risk.
Fragmentation: liquidity and users are spread across many layer 2 networks.

Always check which network a wallet is using before sending funds, because the same address format is used on Ethereum and most EVM rollups.

Frequently Asked Questions

4 questions

What is a layer 2 in crypto?

It is a network that runs on top of a main blockchain, processing transactions more cheaply and quickly and then settling the results on the main chain.

What is the difference between optimistic and ZK rollups?

Optimistic rollups assume batches are valid and allow a challenge period for fraud proofs; ZK rollups prove every batch with cryptography, so no challenge period is needed.

Are layer 2s as secure as Ethereum?

Rollups are designed to inherit Ethereum's security, but many still depend on centralized sequencers or upgrade keys, so their actual guarantees vary.

Does Bitcoin have layer 2s?

Yes. The Lightning Network is a payment-channel layer 2 on Bitcoin, and other Bitcoin layer 2 designs are being developed.

Related Terms

What is a Layer 1 Blockchain Protocol?
A Layer 1 blockchain is the base network that records and settles transactions directly on-chain and runs its own native coin. It owns its security, ledger, and block production, and serves as the foundation that smart contracts, decentralized applications, tokens, and scaling layers are built upon. Bitcoin, Ethereum, Solana, and Cardano are all Layer 1 protocols. The defining challenge for any Layer 1 is the blockchain trilemma — balancing decentralization, security, and scalability, since maximizing two often means compromising the third. Layer 1s scale through consensus design and sharding, or by offloading volume to Layer 2 solutions built on top of them.
What is a Layer-3 Blockchain?
A Layer-3 blockchain is an aggregation and interoperability layer built above Layer-2 networks. Rather than adding raw throughput like a Layer-2, it unifies many independent rollups into one coordinated ecosystem. Layer-3 provides native cross-chain messaging, consolidated liquidity, aggregated proof settlement to Layer-1, and app-specific customization. It tackles five Layer-2 bottlenecks: heterogeneity, liquidity fragmentation, weak communication, inefficient Layer-1 usage, and limited customizability. Leading designs include ZKsync ZKchains, Polygon Supernets, Optimism Superchains, and Arbitrum Orbit. By aggregating proofs and enabling native interoperability, Layer-3 underpins the appchain thesis, horizontal scalability, and chain abstraction across Web3.
What is Arbitrum?
Arbitrum (ARB) is an Ethereum Layer 2 optimistic rollup network developed by Offchain Labs, holding the highest TVL among all Ethereum L2s. Distinguished by its Nitro optimization, Stylus multi-language smart contract support, and BoLD permissionless validation system, Arbitrum is at the forefront of institutional adoption with a $13–20 billion DeFi ecosystem and RWA tokenization.
What is Optimism?
Optimism (OP) is an Ethereum Layer 2 protocol that scales Ethereum via optimistic rollup technology and hosts the Superchain ecosystem — home to over 40 production chains including Coinbase's Base — built on its open-source OP Stack framework.
What is Zero-Knowledge Proofs and zkSNARKs?
A zero-knowledge proof (ZKP) is a cryptographic method that lets one party, the prover, convince another party, the verifier, that a statement is true without revealing any information beyond the validity of that statement itself. Every ZKP must satisfy three properties: completeness (a true claim is accepted), soundness (a false claim is rejected with near-certainty), and zero-knowledge (the verifier learns nothing extra). In crypto, ZKPs power confidential transactions and Layer 2 scaling. zkSNARKs and zkSTARKs are the two dominant proof systems, producing tiny proofs that blockchains can verify far faster than re-running the original computation.