To make clients fetch their packages off the mirror, you need to know its IP number (or its hostname). On a freshly installed Ubuntu Precise, replace the hostname of the repository server with the hostname or IP number of the mirror. E.g. if /etc/apt/sources.list looks like:

###### Ubuntu Main Repos
deb http://ce.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse 

###### Ubuntu Update Repos
deb http://ce.archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse 
      

and your mirror is at 10.0.25.1, then you want to do: