What you mean with "ethernet signal"? I guess, you are referring to twisted-pair copper media and the most popular 10BASE-T and 100BASE-T physical layers, because at least 1000BASE-T exceeds the speed range of TTL. All BASE-T standards are using a return-to-zero (RZ) signaling, in so far they can't be represented by a single binary logic level signal, you would at least need two signals, or do a kind of recoding. A two level comparator should be able to decode the ethernet signal in simple cases (restricted cable length), but not achieving the performance of a dedicated ethernet PHY.