# Chain ID
Learn about the Evmos chain-id format
# Official Chain IDs
# 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 (opens new window). However, in order to comply with both EIP155 and Cosmos standard for chain upgrades, Evmos-compatible chains must implement a special structure for their chain identifiers.
The Evmos Chain ID contains 3 main components
- Identifier: Unstructured string that defines the name of the application.
- EIP155 Number: Immutable EIP155 (opens new window)
CHAIN_IDthat 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.
The format for specifying and Evmos compatible chain-id in genesis is the following:
The following table provides an example where the second row corresponds to an upgrade from the first one:
|ChainID||Identifier||EIP155 Number||Version Number|