Impersonate user in web.config

Split off from http://forums.asp.net/t/1227975.aspx/1?Impersonate+user+in+web+config
Hi All,
  Thanks for your reply.   
  My problem solved after placing the below tag in Web.Config file
<identity impersonate="true">
  Is it necessary to give username and password in that tag. If so, it refers to different server each time so how can i update dynamically?
thanks
gilli
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