Problems installing the kernel-devel package with Yum (FC 5)

["Bart De Soete" <bart de soete gmail com>]

I need to install the kernel-devel with Yum.
But this is what I've got:

[root localhost bdesoete]# yum install kernel-devel
Loading "installonlyn" plugin
Setting up Install Process
Setting up repositories
core                                                                 [1/3]
Cannot find a valid baseurl for repo: core
Error: Cannot find a valid baseurl for repo: core
[root localhost bdesoete]#

What does it means and what can I do about it?
Kind regards