rcn Corp., Princeton, NJ, has signed agreements to buy Boston and Washington, DC?s, largest Internet service providers?UltraNet Communications Inc. of Boston and Erol?s Internet Inc. of Springfield, VA?for a combined $110.5 million in cash and rcn common stock. David McCourt, rcn?s chairman and chief executive, says the company is excited at the prospect of moving its new Internet customers to rcn?s fiber-optic network and offering them 10-Mbit/sec Internet connections and telephone and cable-TV services.