Maybe that will help you to understand:

"Figure 8: Gnutella Servents with Viral Networking
When a user joins the Gnutella network, their Gnutella user program (called a servent because it is both a server and a client) connects to one or more Gnutella PCs. This creates a mesh of connectivity among the Gnutella users.
When a new user PC joins the network, it transmits an introductory “ping” message to its “direct connects,” that is, to the user PCs to which it connects directly. These direct connects pass the message on to the user PCs to which they connect, and so forth. This viral networking continues for several hops, usually five to seven. The user PCs that can be reached in these few hops constitute the Gnutella user’s horizon. In effect, each user sees a different Gnutella network. In fact, they see a somewhat different Gnutella network every time they use it.
Typically, each user can see about 10,000 hosts [Kan, 2001]. This limited horizon prevents traffic from becoming overwhelming as the number of Gnutella users grows. The traffic for each user stays within his or her Gnutella cluster.
All computers reached by a ping message send back a “pong” message giving information about themselves. Pong messages travel back the along the path by which each ping arrived. In other words, each pong is routed along a single path rather than being broadcast as pings are. This reduces traffic.
Pings and pongs allow a new host to estimate its horizon and also to identify the IP addresses of additional hosts to which it may connect [Rohr, 2001]. Connecting directly to multiple hosts improves the speed of viral networking."

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