Since you're probably running TCP/IP this would probably be more specific to your firewall setup (if there is one there) or a route problem.
If you can reach the other labs via TCP using other services like http, ftp, ssh, etc. then the only suggestion I can offer is that you re-examine how smb services are set up on both ends. You may need to set up a machine account if your're using an NT server as a primary domain controller.
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