Sites not working on Some ISPs

There are some sites on one of our Linux servers which are not working on some ISPs in India but I can browse some sites hosted on the same server on those ISPs. I tried opening that site on 5 different ISPs. I can browse those sites on 2 ISPs but but on other 3 ISPs the sites are not working (Error - cannot find server). The sites which are not working can be opened on other parts of world.

I checked everything that I can but could not resolve this issue. Any ideas what could be the reasons? Any suggestions?

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Following example will make the issue more clear:

Suppose I have 2 sites - 'A' and 'B' on the same server.
and there are 5 ISPs (All from India) - 1, 2, 3, 4 and 5

Situation 1 - I can browse site 'A' on 1, 2, 3 ISP but on 4 and 5 it says, cannot find server.

Situation 2 - I can browse site 'A' and 'B' on all the ISPs.

 

 

 

 

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