my webService returns "The request failed with HTTP status 401: Unauthorized"

I wrote a web services with Windows authentication, when i consume this WS from my app inside my network all work great, but when I consume this WS outside my network the WS  return "The request failed with HTTP status 401: Unauthorized"
I'd try the URL in an explorer page and all works great.
In my app I use this:
 
webservice.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials;
What alternatively can I use to get credential inside and outside of my net?
Thanks.
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