Question : LAN Connectivity

I have laptop that has an onbard nic (Realtek RTL8168/8111 PCI-e gigibit ethernet). When I plug a network cable (that is CAT6) in the lan port I can see the LAN connection but when I plug in a CAT5 cable I can't see the LAN connection. Can anyone tell me why this is happening?

Answer : LAN Connectivity

Have decided to send laptop back to manufacturer