This idea came after reading https://bitcointalk.org/index.php?topic=114102.00:

Imagine that someone opened a lot of connections with multiple IPs to other nodes. Number of such connections so big that there are a lot of nodes connected only to the attacker. Let's assume the attacker attempts double-spending attack with the following scenario:

1. He chooses a victim and says that he will pay some bitcoins for something.
2. The attacker sends a transaction only to the victim, all other nodes know nothing about the transaction.
3. The attacker solves 6 blocks so the transaction gets 6 confirmations.
4. The victim sees the confirmations and complete its part of the deal.
5. While the attaker was solving his blocks the main blockchain got higher cumulative difficulty coz more than 6 blocks were solved. Even if the victim reconnects to other nodes it will see that non-legit transaction disappeared from the blockchain and the attacker still owns "spent" coins.

Is it possible theoretically and practically? What could be used to prevent such an attack?