Yep to both: you get addresses via pongs and can also use the ones from hosts you download from.

For an example see
SourceForge.net Repository - [phex] View of /phex/trunk/src/main/java/phex/host/CaughtHostComparator.java
(it shows how hosts are sorted to keep the best ones in the host catcher - this is executed extremely often!)

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Then if we want to maintain the lists,such as to check if one host is still active,then do you use the ping/pong to check it?

But I still don't understand the ping/pong affection to the gnutella network.I think there are many ways should using the ping/pong,for eample "keep alive","gain host list",but when and how to use it,such as how and when to gain host addresses by ping/pong ? I can't find some introduce about this.

I see there are a few ways to gain the host lists,such as the x-try,x-untlrapeer-try when handshakeing,the pong is just one of these ways to gain the hosts list to connect to,is it right?So the host cache lists should be gained from all there ways.