checking for new updates

Hello, i am pritam sathe
I have this query related to update manager.
I try to check for new updates in update manager its shows me this :

 (Failed to download repository information

 Check your Internet connection.)

I have active connection............
and when i see details it show this :

W:Failed to fetch cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1)/dists/natty/main/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs,

W:Failed to fetch cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1)/dists/natty/restricted/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs,

E:Some index files failed to download. They have been ignored, or old ones used instead.