Peer-to-peer Networks
Stands for “Peer to Peer.” In a P2P network, the “peers” are computer systems which are connected to each other via the Internet. Files can be shared directly between systems on the network without the need of a central server. In other words, each computer on a P2P network becomes a file server as well as a client.
P2P代表什么意思呢？在一个P2P网络中，对等的网络节点通过网络相互连接。用户（节点）可以不需要通过中心化的服务器去分享文件资源，可以直接从其他节点获取资源。换句话说，在p2p网络中的每一个计算机节点即可以是接受资源的客户端也可以是发送资源的服务端。
The only requirements for a computer to join a peer-to-peer network are an Internet connection and P2P software. Common P2P software programs include Kazaa, Limewire, BearShare, Morpheus, and Acquisition. These programs connect to a P2P network, such as “Gnutella,” which allows the computer to access thousands of other systems on the network.
Once connected to the network, P2P software allows you to search for files on other people’s computers. Meanwhile, other users on the network can search for files on your computer, but typically only within a single folder that you have designated to share. While P2P networking makes file sharing easy and convenient, is also has led to a lot of software piracy and illegal music downloads. Therefore, it is best to be on the safe side and only download software and music from legitimate websites.
Hybrid Peer-to-peer Networks
A hybrid P2P network is one that has an index server containing information on the locations of resources at the center, and which uses the index server for search. A pure P2P network is one that directly executes the search process among hosts without a server between the hosts.
在混合P2P网络中存在一个索引服务器，索引服务器存储着当前网络中各个节点的资源以及对应节点的索引信息。如果客服端需要下载对应资源，需要请求索引服务器查询资源和拥有资源的节点，进而访问资源节点。相比一个纯P2P网络，不需要逐一请求网络中的节点。
References
Distributed hybrid P2P networking systems
https://link.springer.com/content/pdf/10.1007%2Fs12083-014-0298-7.pdf
Data-intensive Systems slides from University of St Andrews
