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I have a Desktop PC (Core i5 processor, GA-B85M-D3H Gigabyte motherboard). I want to expand the system to support 4 ethernet ports. (1 incoming, 3 outgoing ports connected to 3 different ISPs for load balancing).
How do I find out how many ethernet ports can my motherboard support? Official specifications do not talk about it.
There is no specific limit.
On a modern motherboard you have PCI-e slots and optionally a few build in Ethernet ports.
In each PCI-e slot you can add an expansion cards. That can be a traditional cheap NIC with one port, or more server orientated cards. Many of these feature 4 ports.
Thus, without doing anything exotic:
As to exotic: The can use PCI-e to multiple PCI-2 bridges.
Thus far the generic answer. Now for your specific setup:
Your board has one on board NIC, three PCI-e conectors and two ancient PCI-connectors. In theory you could have up to 21 Ethernet ports.
That is assuming no other cards are in use, and using PCI rather than PCI-e is not recommended (it is a shared bus with relative low bandwith, you you can put a quad card in it, but a single 1Gbit NIC can saturate a 33Mhz/32bit PCI bus).
