I had a capital gain of $15,000 last year. This year I had a $100,000 loss. Can I net the two in order to reduce last year’s tax bill?

No, there is no netting or income averaging for capital gains and losses that occurred in different years. You must report last year’s gain, which can only be offset by last year’s capital losses, plus any capital loss carryforwards to last year from prior years. This year’s loss can offset capital gains this year, plus up to $3,000 of ordinary income. Any excess capital loss can be carried forward and used in future years. There’s no time limit for using up capital loss carryovers.