What is the area code for Nassau County NY?

Nassau County, New York, uses telephone area code 516 (international: +1 516). Suffolk County, the eastern part of Long Island, uses area code +1 631. The area codes exactly follow the county boundary.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)