I'll try to explain this in the least confusing way possible.
I'm using my Mac's ethernet connection, and sharing it as Wifi for my DS. I can connect to the normal GTS fine. But I'm trying to connect to Pokecheck. So I change the settings, don't auto detect DNS and put in the DNS on the top of their page. When I test the connection I can get a good 3 seconds of 3 bars/green connection, but then it goes red. I'm assuming my university has Pokecheck's DNS blocked? I don't understand and I really want to try to get around it. I'm surprised they have it blocked or know what it is. I tried running a script on my Mac that tells me if my ISP is blocking it, but it didn't work.