Learn about the Ethermint chain-id format {synopsis}
## The Chain Identifier
Every chain must have a unique identifier or `chain-id`. Tendermint requires each application to
define its own `chain-id` in the [genesis.json fields](https://docs.tendermint.com/master/spec/core/genesis.html#genesis-fields). However, in order to comply with both EIP155 and Cosmos standard for chain upgrades, Ethermint-compatible chains must implement a special structure for their chain identifiers.
## Structure
The Ethermint Chain ID contains 3 main components
- **Identifier**: Unstructured string that defines the name of the application.
- **EIP155 Number**: Immutable [EIP155](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-155.md) `CHAIN_ID` that defines the replay attack protection number.
- **Version Number**: Is the version number (always positive) that the chain is currently running.
This number **MUST** be incremented every time the chain is upgraded or forked in order to avoid network or consensus errors.
### Format
The format for specifying and Ethermint compatible chain-id in genesis is the following:
```bash
{identifier}_{EIP155}-{version}
```
The following table provides an example where the second row corresponds to an upgrade from the first one:
| ChainID | Identifier | EIP155 Number | Version Number |
