How was Ethereum hard fork's replay attack was handled? On etherum, there was a hard fork which resulted in ETH and ETC.  My reading indicates that it suffered problem of replay-attack.

Question:


How was this problem resolved for Ethereum?  In my understanding, replay-attack protection requires another hard fork, because replay-protection inherently requires making transaction incompatible among the 2 chains.  Was such fork operated?  Or does Ethereum still have possibility of replay-attack if transaction is to be made from the very old wallet (the wallet which have no associated transaction after the fork)?
There's some history when it comes to ETH/ETC replay protection.


  How was this problem resolved for Ethereum? 


Initially when ETH hard-forked to invalidate the DAO contract, both ETC/ETH networks did not implement replay protection for transactions. Instead, a split contract was written instead which relies on a split oracle built in Ethereum to identify if the contract was called on the hard-fork (i.e. ETH chain). 

The contract logic is quite simple as you can see for yourself, it basically splits the original tokens and sends them into two different addresses depending on which chain the contract is called. 

Nowadays, the ETH (hard-forked) chain has effectively implemented replay protection. So transactions executed on the ETC network will not be valid on the ETH network and vice-versa.


  Does Ethereum still have possibility of replay-attack if transaction is to be made from the very old wallet (the wallet which have no associated transaction after the fork)?


No.