It depends on the ISP. I'll give you some actual useul info, cuz I don't know know what they're trying to tell you, and maybe you can understand that, but that was pure jibberish to me. Some ISP's have open ports to their proxies. That means that they'll always have open proxies. So for example, you scan an ISP from Brazil named Global Village Telecom you'll always fine some open proxies. You would need to rescan those ranges everyday though, because some of the computers with open ports may not be powered on at that moment, or, they might be DSL lines. In that case the IPs would change every so often. In any event, you're best bet is to build a good list of ranges that you can find good proxies on, and scan them each day. If you're using something like Proxyfire then yes, you'll have to recheck those. My advice though would be to just get a good checker that doesn't require that.
Edit: Meant to mention this, you'll want to filter those IPs as Proxygo has said as well. Depending on what you're doing, you can run into a lot of trouble, or even give away your method by using open-proxies like that, so you definitely need to be careful.
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