Introduced in 2003 the Apple PCI-X Gigabit Ethernet Card Kit (part M9451LL/A) is a PCI-X based gigabit Ethernet card released by Apple Computer based on a Broadcom BCM5703 Ethernet controller. The card uses the (at the time) very fast 133 MHz/64-bit PCI-X interface found in early Power Macintosh G5 computers, which is backwards compatible with PCI slots found in G4 and earlier machines as it will drop to 100, 66 or 33 MHz as needed. I am not 100% sure if it is compatible with 32-bit PCI slots - I will have to test that one day, but it very well could be as its Broadcom Ethernet controller chip does support 32-bit/33 MHz PCI slots. For an Ethernet interface the card has an rj-45 connector for shielded or unsheilded twisted-pair cabling. Apple also produced a similar PCI-X card for Fiber Channel networks.

The relatively high price of almost $100 US seems very far from the generic gigabit Ethernet cards one can purchase for around $10, but generic Ethernet cards tend to use 33 MHz/32-bit PCI interfaces instead of 133 MHz/64-bit PCI-X, which has a much higher transfer rate. Although technically the 133 MB/sec 33 MHz/32-bit PCI interface used on cheaper cards is still faster than the theoretical 125 MB/sec maximum for gigabit Ethernet it leaves little headroom for other cards.

This card supports jumbo frames under OS X. Jumbo frames are frames larger than the usual 1500 bit packets (usually 9000) which can improve performance in most cases.