Can someone tell me what happen after the hardfork (due to DAO attack)? There are 2 ethereum cryptos after the fork, so there should be 2 different network. Are they using the same network ID? How do they differentiate themselves?
Apology if I am asking something basic. Appreciate if someone can point me to some Ethereum ecosystem primer...
ETC has the same network_id = 1 as ETH (see this question for the list of known network ids).
As ETH and ETC nodes use the same wire protocol they can connect to each other and send blocks and transactions. But ETC and ETH have different sets of bootnodes predefined in their clients. This results in two non-overlapping networks.
To distinguish ETH and ETC transactions chain_id was introduced in EIP-155. chain_id is mixed into transaction signature to prevent replay attacks.
So, if ETH node somehow connects to ETC peer it will get block or transaction that does not belong to its network but it will consider those as invalid and will drop connection to that peer.
The individual nodes in the network for Ethereum and Ethereum Classic run different software which is not interoperable. This is how a hard fork is created, when a change is made to the protocol which prevents it from interacting with the protocol before the change. If the nodes in the network don't update their software they will continue running the old protocol (ETC) and if they do update, they will run the new protocol (ETH). The difference is in the software used, not the network ID.