Map of Zip Code 11791 Border. Zip Code 11791 is located in the state of New York in the Metro New York City metro area. Zip code 11791 is primarily located in Nassau County. The official US Postal Service name for 11791 is SYOSSET, New York. Portions of zip code 11791 are contained within or border the city limits of Syosset, NY, Muttontown, NY, Laurel Hollow, NY, Oyster Bay Cove, NY, Hicksville, NY, Woodbury, NY, Cold Spring Harbor, NY, Jericho, NY, Plainview, NY, East Norwich, NY, and Brookville, NY. The area code for zip code 11791 is 516.