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---
caip: 2
title: Blockchain ID Specification
author: Simon Warta (@webmaster128), ligi <ligi@ligi.de>, Pedro Gomes (@pedrouid)
discussions-to: https://github.com/ChainAgnostic/CAIPs/pull/1
status: Final
type: Standard
created: 2019-12-05
updated: 2023-01-25
---
# CAIP-2: Blockchain ID Specification
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> **Source:** [ChainAgnostic/CAIPs](https://github.com/ChainAgnostic/CAIPs/blob/main/CAIPs/caip-2.md)
>
> This document is used by x402 for cross-chain network identification.
## Simple Summary
CAIP-2 defines a way to identify a blockchain (e.g. Ethereum Mainnet, Goerli, Bitcoin, Cosmos Hub) in a human-readable, developer-friendly and machine-readable format.
## Abstract
Often you need to reference a blockchain to provide context to data, messages, assets, accounts, etc. This CAIP describes a way to identify blockchains. Blockchains are different than a specific network which is covered by [CAIP-4](https://github.com/ChainAgnostic/CAIPs/blob/main/CAIPs/caip-4.md).
## Motivation
Currently, there is no standard for how to identify a blockchain. In some cases, they are identified by their genesis hash, in others by a numeric chain ID, and in others by a string representing the chain name. This CAIP offers a way to uniquely identify a blockchain in a human-readable, developer-friendly and machine-readable way.
## Specification
The blockchain ID is a string designed to uniquely identify a blockchain. The format is:
```
chain_id: namespace + ":" + reference
namespace: [-a-z0-9]{3,8}
reference: [-_a-zA-Z0-9]{1,32}
```
### Namespace
The namespace represents a class of similar blockchains. It's usually based on the underlying blockchain technology or virtual machine.
Examples:
- `eip155` - EVM-compatible chains (Ethereum, Polygon, Arbitrum, Base, etc.)
- `bip122` - Bitcoin and Bitcoin-like chains
- `cosmos` - Cosmos SDK chains
- `solana` - Solana clusters
- `polkadot` - Polkadot and Kusama networks
### Reference
The reference is the specific blockchain or network within a namespace. For EVM chains, this is typically the Chain ID as defined in [EIP-155](https://eips.ethereum.org/EIPS/eip-155).
## Semantics
Each `namespace:reference` pair MUST refer to one and only one blockchain.
## Rationale
The goals of the identifier format are:
- Human readability and developer-friendliness
- Machine-readable
- Low collision risk
## Examples
### EVM Chains (eip155 namespace)
| Chain | CAIP-2 ID |
|-------|-----------|
| Ethereum Mainnet | `eip155:1` |
| Ethereum Goerli | `eip155:5` |
| Ethereum Sepolia | `eip155:11155111` |
| Polygon Mainnet | `eip155:137` |
| Polygon Mumbai | `eip155:80001` |
| Arbitrum One | `eip155:42161` |
| Arbitrum Sepolia | `eip155:421614` |
| Base Mainnet | `eip155:8453` |
| Base Sepolia | `eip155:84532` |
| Optimism Mainnet | `eip155:10` |
| BNB Smart Chain | `eip155:56` |
| Avalanche C-Chain | `eip155:43114` |
### Bitcoin (bip122 namespace)
| Chain | CAIP-2 ID |
|-------|-----------|
| Bitcoin Mainnet | `bip122:000000000019d6689c085ae165831e93` |
| Bitcoin Testnet | `bip122:000000000933ea01ad0ee984209779ba` |
### Solana (solana namespace)
| Cluster | CAIP-2 ID |
|---------|-----------|
| Solana Mainnet | `solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp` |
| Solana Devnet | `solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1` |
| Solana Testnet | `solana:4uhcVJyU9pJkvQyS88uRDiswHXSCkY3z` |
### Cosmos (cosmos namespace)
| Chain | CAIP-2 ID |
|-------|-----------|
| Cosmos Hub | `cosmos:cosmoshub-4` |
| Osmosis | `cosmos:osmosis-1` |
## Usage in x402
x402 uses CAIP-2 identifiers in the `network` field of payment requirements:
```typescript
{
"accepts": [{
"scheme": "exact",
"network": "eip155:84532", // Base Sepolia
"price": "$0.001",
"payTo": "0x..."
}]
}
```
This allows x402 to support multiple chains and networks in a standardized way.
## Test Cases
```
# Valid blockchain IDs
eip155:1 # Ethereum Mainnet
eip155:8453 # Base Mainnet
eip155:84532 # Base Sepolia
solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp # Solana Mainnet
bip122:000000000019d6689c085ae165831e93 # Bitcoin Mainnet
cosmos:cosmoshub-4 # Cosmos Hub
# Invalid blockchain IDs
1 # Missing namespace
eth:1 # 'eth' not a valid namespace
eip155: # Missing reference
```
## References
- [EIP-155: Simple replay attack protection](https://eips.ethereum.org/EIPS/eip-155)
- [CAIP Repository](https://github.com/ChainAgnostic/CAIPs)
- [Chain Agnostic Standards Alliance](https://github.com/ChainAgnostic/CASA)
## Copyright
This CAIP is licensed under [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
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