The Realtek RTL8139 is a family of network adapters which came out in 1999. Starting with the single chip 8139 follows versions A,B,C,D each with various new technologies and features added such as Wake from LAN. Even though Network adapters and wireless have superseded adapters like this they are still an amazingly reliable way to connect to the a Network or the Internet. For retro gamers and computer historians that absolutely have to have original hardware, this is the standard.
Recent Operating Systems going back to 2007 pretty much have the drivers for this device, if it is not detected the card could be faulty.