On systems built on the HP ProLiant DL380, there are three ethernet ports: two built into the motherboard and one on a PCI card (which it shares with a video port and a PS/2 port). The two built into the motherboard are standard interfaces for the server: in most cases you should only have to plug in port 1. The ethernet interface on the PCI card is for the remote management board which allows out-of-band access to the server. You'll need to plug this interface in to remote manage the server.
On systems built on the HP ProLiant ML330, there are two ethernet ports: one that shares a PCI slot with a single video port and another one that shares a PCI slot with a video port and a PS/2 port. The former is the system's network interface. The latter is the interface for the remote management board (as described in the previous paragraph).
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