ISA slots aren't active by default. You need some sort of bridgeboard to activate them... like GoldenGate or some hardware PC-emulator. Then you could use ne2k compatible isa-card, but that isn't necessarily the best solution.
Better way is to get some Zorro card, like X-surf, Ariadne 1 or 2, Hydra or some Commodore's network card. X-surf and Ariadne2 are the latest and recommendable. Older ones might have some problems like with Hydra's Amiganet 1.1, which need little hardware modification to get it work on A4k...
