Computers show as a mixture of ip address or name



Hello.
 
When i go to Administrator Console/Computer Management/System Tools/Shared Folders/Sessions, it shows all of the computers logged in.  However some of the computers show with a computer name and others show an ip address.
 
Is there a reason why they are mixed like that ?
 
They all join the domain when they turn on their computers.
 
SBS2000, SP4, all updates and patches.
 
Any help would be greatly appreciated.
 
Thanks,
Tony