updates - Why does apt-get give me an "Unable to connect" error?

After installing Ubuntu 12.04, I tried to update, but I got the following:

student@student-25:~$ sudo apt-get update && sudo apt-get upgrade
Ign cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release i386 (20120423) precise InRelease
Ign cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release i386 (20120423) precise/main TranslationIndex
Ign cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release i386 (20120423) precise/restricted TranslationIndex
Ign cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release i38Translation-en
Ign http://extras.ubuntu.com precise InRelease                                 
Err http://extras.ubuntu.com precise Release.gpg                               

  Unable to connect to extras.ubuntu.com:http:

Help?