The Layer 1 (L1) Foundation
A Layer 1 blockchain is the main, underlying network architecture (e.g., Bitcoin, Ethereum, Solana, Avalanche). It is responsible for its own consensus mechanism (Proof of Work/Stake), data availability, and transaction settlement.
The problem? The "Blockchain Trilemma" dictates that a network can only optimize two of three properties: Decentralization, Security, and Scalability. Ethereum prioritized extreme Security and Decentralization, meaning it handles relatively few transactions per second (TPS), causing brutal gas fees during high network demand.
The Layer 2 (L2) Solution: Rollups
A Layer 2 is a secondary blockchain built explicitly on top of an L1 (like Ethereum). Examples include Arbitrum, Optimism, and Base.
Instead of executing every transaction on the slow L1, the L2 processes thousands of transactions off-chain, compresses them into a single mathematical proof (a "Rollup"), and posts that tiny batch of data back down to the L1.
This allows the L2 to offer instantaneous speed and microscopic fees, while still borrowing the unhackable security guarantees of the underlying Ethereum L1.
Monolithic vs Modular Visions
This creates two distinct philosophies in crypto. The "Modular" approach (Ethereum) believes the L1 should only act as a slow, ultra-secure settlement vault, while everyday users only interact with swift L2 networks.
The "Monolithic" approach (Solana) believes the entire system—execution, consensus, and data—should happen on a single lightning-fast L1 layer to prevent user fragmentation and complexity, accepting higher hardware demands as a trade-off.