NEW YORK — Cisco Systems Inc., the world’s largest maker of computer networking gear, warned Wednesday that orders fell off abruptly in October and projected a large fall in sales in the current quarter. Its shares slid in after-hours trading. On a conference call with investors after Cisco reported flat earnings for the previous quarter, Chief Executive John Chambers said he expects revenue to fall 5 percent to 10 percent in the current quarter, which began Oct. 26.