Back when the Quadras and Centrises were shiny and new, ethernet was still kind of rare and expensive, found more often in institutions than in homes and came in three different physical media flavors. There were two forms of ethernet media on coax cable, thick net and thinnet, and there was 10base-T running on UTP, unshielded twisted pair cable.

As we know now, UTP won the cable wars for ethernet. But that wasn't so clear back in the early 90s.

So, there was a type of port called AUI, Attachment Unit Interface which used a DA-15 connector. The AUI brought out connections for the Media Access Control and provided an interface to which to connect the MAU (Medium Attachment Unit) or transceiver.

Depending on what type of transceiver you attached, you could connect to any of the three flavors of ethernet physical media used back then. At a time when an ethernet interface cost a couple of hundred dollars, this versatility was nice.

Well, when Apple first added ethernet to the Macintosh, they decided to use an AUI interface, which was how the Unix'y guys and workstations did things. So that was kind of nice. But they decided (I think) that the DA-15 connector was too much like the Macintosh's display-out connector. Folks were too likely to plug their Apple monitor into the AUI.

So, Apple invented AAUI (Apple Attachment Unit Interface). Which is the funny little connector you see on the back of your Quadra. And, as someone else pointed out, you need a transceiver in order to use it.

Make sure you get a UTP or 10base-T or RJ45 transceiver. There may still be thinnet and thicknet transceivers out there. I bought some 10base-T transceivers about ten years ago and thinnet was in the boxes.