isc domain lookup

[Yossi Eskenazi]

Hello,

My question is not directly related to bind, but since it's related to isc
I decided to ask here.

Some name servers I perform a lookup does not respond to queries, if they
begin with "isc"

Example server : 213.143.254.38
Query for any domain, it responds. Sometimes it times out, but ask again ,
it eventually responds.
Query for "isc.com" "isc.org" "isc.whatever.it.is" or just "isc",  it times
out.

Example server: 185.15.40.80
Query for any domain, it says "query refused"
Query for "isc.com" "isc.org" "isc.whatever.it.is" or just "isc",  it times
out.

It's not just these two, there are other name servers behaving this way.

What can be the reason for this?

Thanks for your help

Y.E.
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