Reporting the Basis of Stock Sold

Since 2010 brokers have been tracking your cost basis and reporting it to the IRS when you sell corporate stock. No problem there. What about stock you acquired prior to 2010? You have to keep records proving the basis of these stocks. The IRS says that if you can't prove basis, then the basis is zero which means all of your gross proceeds are taxable gains. What about if you sell less than your entire holding of a corporate stock? There are only two methods to determine basis of stock sold under this circumstance: first in first out (FIFO) which means the stock is considered to be sold in the order it was purchased, and the specific identification method where the seller tells the broker the exact stock to be sold by certificate number.