Understanding Network Ports
You need to understand three important concepts
about network ports. A port must be active and configured to be used for networking, and each
port has a priority that is either
automatically or manually assigned.
An active port is one that has been enabled in
Network preferences, and then connected to its network type:
AirPort connects to a local wireless network; Ethernet requires a
network cable with a responder of some sort; the modem requires a
dial tone. The configuration might be as simple as selecting a base
station or setting a TCP/IP address, but it is an essential part of
the port definition.
If you have two or more active ports, the
computer needs to know which port should be used to send nonlocal
network data. For example, if you have a computer connected to both
an Ethernet and an AirPort network, and you attempt to access an address not found on either
network, the operating system needs to know which network contains
the router that should be used to access the WAN. Choose Network
Port Configurations from the Show pop-up menu to set port
priority.
When the computer needs to send network packets,
it sends through the active port with the highest priority. The
higher the port in the list, the higher its priority. In the
Network Port Configurations pane above, both the Built-in Ethernet
and AirPort port configurations are enabled. If both ports are
active and connected to networks, any packets would be sent to the
router on the Ethernet network, because the port configuration
Built-in Ethernet has the highest priority.
Mac OS X is designed to change its highest
priority network port automatically to maintain valid network
connections. If your Ethernet cable is unplugged or fails, or if
your computer loses Internet connectivity due to a server problem,
Mac OS X would automatically attempt to make AirPort the primary
network port. If Internet services are restored on the Ethernet
interface, Ethernet will become active again, and automatically
receive its former priority.
NOTE
If you have two or more active ports, the DNS
servers associated with the port with the highest priority are used
for domain name conversion. If you are having problems locating
servers on a network connected to a lower priority port, those
services might not be accessible by the DNS servers on the highest
priority port. For example, in the preceding screenshot, if both
the Ethernet and AirPort ports are active, DNS lookups will occur
using the DNS servers listed with the Ethernet port. If the AirPort
network is a private network, your computer will not be able to
perform a domain name lookup for services on that network, even
though your computer is directly connected to it.
Enabled ports are listed in the Show pop-up menu
in Network preferences. Choosing a port from the Show pop-up menu
allows you to configure the port.
Which Network configuration panes are available
depends upon which port is chosen in the Show pop-up menu. The
order of the panes corresponds with the most likely use for that
type of port. When an Ethernet port is chosen, five panes are
available: TCP/IP, PPPoE, AppleTalk, Proxies, and Ethernet. When
AirPort is chosen, the five panes are AirPort, TCP/IP, PPPoE,
AppleTalk, and Proxies. With modems, the available panes are PPP,
TCP/IP, Proxies, and Modem.
 
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