The 6100 had an AAUI ethernet port available (10BaseT) that would work fine for you. All you need is the AAUI to RJ45 interface. These 'dongles' can be had for about $15-20 online or look at your local apple vendor (one that deals with new and older hardware preferrably) and ask them about getting one.
I have a 7100, and it runs fine as a server with MkLinux through the port. It plugs into my ethernet hub, then to a router without a hitch.
You say you're running a server now through localtalk? Do you have a software bridge to route the internet TCP/IP packets onto MacIP? If its a hardware bridge you may not have the ability to use the localtalk interface ability that you get when running a MacOS on the hardware.
I didn't know NetBSD worked on this class of a machine. I thought you needed an openfirmware available mac (and these machines have a sort of pre-openfirmware, firmware). At least for the PPC version. There are 68k versions that work on even older hardware.
Good luck, and I'm off to check the specs required for NetBSD!
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http://www.netbsd.org/Ports/macppc/models.html
Looks like its still unsupported. An alternative is to try and use MkLinux as a springboard and then get the monolithic linux working by replacing the kernel with a few supplied here.
Other distributions may work, I just think they'd have to be manually installed (not for the faint of heart) and impractical for this age of hardware.
Look at:
http://nubus-pmac.sourceforge.net/
[ 03-28-2002: Message edited by: bluedog ]