Area Code 301-231 located in ROCKVILLE, MONTGOMERY, MD, more detail is as below. Place NPA (Numbering Plan Area) : 301 NXX (Local Number Prefix) : 231 City : ROCKVILLE County : MONTGOMERY State : MD (Maryland) NXX Use Type : Landline NXX Intro Version : 1994-02-04 FIPS (Federal Information Processing Standards) : 24031 LATA (Local Access Transport Area) : 236 Rate Center : WSNGTNZN11 ZIP Code Count : 117 ZIP Code Frequency : 7 ZIP Code : 20851 Other Information NPANXX : 301231 County Population : 0 Switch CLLI (Common Language Location Identifier) : RKVLMDMRDS0 MSA CBSA (Core Based Statistical Area) : Washington-Arlington-Alexandria, DC-VA-MD-WV MSA CBSA Code (Core Based Statistical Area) : 47900 Time Zone : Eastern (GMT -05:00) Observes Daylight Saving Time : Yes Company : VERIZON MARYLAND, INC. OCN (Operating Company Number) : 9212 Coverage Area Name : ROCKVILLE Latitude : 39.0558 Longitude : -77.1230 ‹ previous : 301-230 next › : 301-231 Telephone Number List 301-231-0000 ~ 301-231-9999