While not "big" by any sense of the word, we NAT [almost] all of our internal network. It wasn't initially a matter of choice, but rather of necessity. We had a sprinklings of small netblocks in the old legacy C swamp, mostly in the old SURAnet/BBN allocation, and after the Genuity takeover they yanked our routes on short notice (actually, our upstream didn't notify us until the last minute). We had to NAT into a new temporary allocation from an upstream, and later renumbered into a portable block for multihoming. There are still some old Genuity addresses in use inside (renumbering is easier said than done) but we're slowly cleaning them up. NAT seemed to be the best option at the time, especially since we had no portable allocation. We used to overload, but talk about overhead... Jeff