IIRC the IP to IP transaction system would have you enter an IP address and then your node would send a message to that IP address asking it for a public key. The node at that IP address would then send back to your node the public key. Then that public key is used to create a Pay-To-Public-Key output in a transaction. The transactions themselves are the same transactions that we are familiar with today, just that instead of the receiver telling you an address, the nodes would do it themselves. This obviously has some privacy implications and likely some vulnerabilities so it was removed.