question about TransmitFile function.

I am now useing "TransmitFile " function.
it is ok when i download my computer's file.
for example) response.TransmitFile("c:\test.txt");

but if i use ip address, it does not work.
for example) response.TransmitFile("http://xxx.xx.xx.xx/test/test.txt");

how can i solve this problem?
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