can not access site collection using IP of web server

HI

when i create new site collection in web application i can access  using machiine name but not access using IP

in side server and outside server in same doomain.

http://10.xxx.0.20/sites/courtstest/default.aspx

when i use 

http://testenvspt/sites/courtstest/default.aspx i can acess  from inside server but from outside not

it shows webpage can not found

adil