> ## Documentation Index
> Fetch the complete documentation index at: https://docs.optimism.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Interoperability predeploys

> Learn how interoperability predeploys work.

<Info>OP Stack interop is in active development. Some features may be experimental.</Info>
The following predeploys have been added to enable interoperability.
*Predeployed smart contracts* exist at predetermined addresses, coming from the genesis state.
They're similar to [precompiles](https://www.evm.codes/precompiled) but run directly in the EVM instead of running as native code.

## CrossL2Inbox

The `CrossL2Inbox` is the system predeploy for cross chain messaging.
Anyone can trigger the execution or validation of cross chain messages, on behalf of any user.

* **Address:** `0x4200000000000000000000000000000000000022`
* **Specs:** [`CrossL2Inbox`](https://specs.optimism.io/interop/predeploys.html?utm_source=op-docs\&utm_medium=docs#crossl2inbox)
* **Source code:** [`CrossL2Inbox`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/src/L2/CrossL2Inbox.sol)

## L2ToL2CrossDomainMessenger

The `L2ToL2CrossDomainMessenger` is a higher level abstraction on top of the `CrossL2Inbox` that provides general message passing.
It's utilized for secure ERC20 token transfers between L2 chains.
Messages sent through the `L2ToL2CrossDomainMessenger` on the source chain receive both replay protection and domain binding (the executing transaction can only be valid on a single chain).

* **Address:** `0x4200000000000000000000000000000000000023`
* **Specs:** [`L2ToL2CrossDomainMessenger`](https://specs.optimism.io/interop/predeploys.html?utm_source=op-docs\&utm_medium=docs#l2tol2crossdomainmessenger)
* **Source code:** [`L2ToL2CrossDomainMessenger`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/src/L2/L2ToL2CrossDomainMessenger.sol)

## SuperchainETHBridge

The `SuperchainETHBridge` is a predeploy contract that facilitates cross-chain ETH bridging within the OP Stack interop cluster. It serves as an abstraction layer on top of the `L2ToL2CrossDomainMessenger` specifically designed for native ETH transfers between chains. The contract integrates with the `ETHLiquidity` contract to manage native ETH liquidity across chains, ensuring seamless cross-chain transfers of native ETH.

* **Address:** `0x4200000000000000000000000000000000000024`
* **Specs:** [`SuperchainETHBridge`](https://specs.optimism.io/interop/superchain-eth-bridge.html)
* **Source code:** [`SuperchainETHBridge`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/src/L2/SuperchainETHBridge.sol)

## ETHLiquidity

The `ETHLiquidity` contract is a predeploy that manages native ETH liquidity for cross-chain transfers within the OP Stack interop set. It works in conjunction with the `SuperchainETHBridge` to facilitate the movement of ETH between chains without requiring modifications to the EVM to generate new ETH.

The contract is initialized with a very large balance (type(uint248).max wei) to ensure it can handle all legitimate minting operations. This design allows the `SuperchainETHBridge` to have a guaranteed source of ETH liquidity on each chain, which is essential for the cross-chain ETH transfer mechanism.

* **Address:** `0x4200000000000000000000000000000000000025`
* **Specs:** [`ETHLiquidity`](https://specs.optimism.io/interop/eth-liquidity.html)
* **Source code:** [`ETHLiquidity`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/src/L2/ETHLiquidity.sol)

## SuperchainTokenBridge

The `SuperchainTokenBridge` is an abstraction on top of the `L2ToL2CrossDomainMessenger` that facilitates token bridging using interop, as described in the [token bridging spec](https://specs.optimism.io/interop/token-bridging.html).

* **Address:** `0x4200000000000000000000000000000000000028`
* **Specs:** [`SuperchainTokenBridge`](https://specs.optimism.io/interop/predeploys.html?utm_source=op-docs\&utm_medium=docs#superchainerc20bridge)
* **Source code:** [`SuperchainTokenBridge`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/src/L2/SuperchainTokenBridge.sol)

## Next steps

* Learn [how messages get from one chain to another chain](/app-developers/guides/interoperability/message-passing)