This chapter provides detailed of attacks on Peer-to-Peer network with their suitable defensive technique or algorithm. We try to reduce the effect of Sybil attack from p2p network. In peer-to-peer (p2p) network is a way of structuring distributed applications such that the individual nodes have symmetric roles. Peer-to-peer network having no any hierarchy that means all peers are equal and no administrator responsible for the network. There are a several kinds of attacks in peer-to-peer network. Like, DOS Attack, DDOS attack, Rational attack, Sybil attack and Eclipse attack. In Sybil attack, Attacker create Fake Identities on p2p network either to gain better reputation or increase download capabilities or take entire control over network. A faulty node may present multiple identities to a peer-to-peer network and then after becoming part of the p2p network and act maliciously. By multiple identities, the faulty node can control the network substantially. By using Reputation ranking algorithm after efficient changing we can check the reputation rank of all the peers connected to that network. After checking the reputation rank we decide which nodes are real and which nodes are fake. That means aim of reducing Sybil attack is done from the following proposed flow and algorithm.