A chain ID is a number that identifies an EVM network, such as Ethereum or Arbitrum One. For a cross-chain transfer, the source ID and destination ID must stay attached to the correct parts of the route.
Which identifiers need to match?
Each step must name the chain it uses, because an address or token symbol alone cannot identify a network.
- Source chain ID
- Destination chain ID
- Token address paired with its chain
The source chain ID tells a wallet where to send and sign the first transaction. Ethereum’s ID is 1; Arbitrum One’s is 42161.
The destination chain ID tells the bridge where to complete the transfer. These values are separate: an Ethereum-to-Arbitrum route uses 1 as its source and 42161 as its destination.
A token address identifies a contract only within its own chain. The same address can exist on two networks and refer to different contracts, so record a token as its chain plus its address.
How do IDs prevent a mismatch?
A wallet reads the active network’s ID through eth_chainId, a standard request to a blockchain node. When signing a transaction, the wallet includes the chain ID so the signature is intended for that network; this also helps prevent replay, where someone resubmits a transaction on another chain.
For a quick example, say you want to move a token from Ethereum to Arbitrum One. The source transaction should be signed for chain 1, while the route’s destination should say 42161; swapping those values can send a request to the wrong network or make it fail.
Bungee Bridge, built by Socket, finds routes across bridges and exchanges for transfers between EVM networks and layer 2 chains. If you are comparing routes for this task, the Bungee Bridge aggregator is one way to handle it; the chain IDs still describe which networks each route connects.
For an app or developer checking IDs, compare the wallet’s active chain with the route’s stated source, then check the destination separately. Socket API and Socket Gateway are examples of Socket tools used in cross-chain routing, where keeping source and destination details distinct matters.
One edge case: an ID is a label, not proof that a network or RPC endpoint is trustworthy. If two networks share an ID, the number alone cannot tell them apart, so verify the network through a trusted source as well.
CAIP-2 is a shared format that writes a chain as a namespace and reference, such as eip155:42161. It can make network names clearer across apps, but the same rule holds: check the source, destination, and token’s chain together.
In practice, match each chain ID to its role in the route, and pair every token address with its chain. That simple check catches common cross-network mix-ups before you sign.