PLT OF BL8 ENTERS DC ADIZ AFTER RADAR SERVICE IS TERMINATED FROM NHK APCH CTLR.

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FLT WAS A XCOUNTRY FLT FROM 2W6 ARPT IN SOUTHERN MARYLAND TO MSV ARPT IN SE NEW YORK STATE. RTE OF FLT WAS PLANNED TO PASS THROUGH THE E SIDE OF THE WASHINGTON ADIZ. AN ADIZ FLT PLAN WAS FILED WITH LEESBURG FSS. ON DEP FROM 2W6 (OUTSIDE ADIZ); PLT CONTACTED PATUXENT APCH FOR ADVISORIES AND FLT FOLLOWING. A REQUEST WAS MADE WITH PATUXENT APCH TO OBTAIN MY IFR SQUAWK FROM POTOMAC APCH. CTLR RESPONDED WITH 'I'LL SEE WHAT I CAN DO.' MOMENTS LATER; THE CTLR GAVE ME A SQUAWK CODE AND THE STANDARD 'RADAR CONTACT.' AS I PROCEEDED N TOWARD THE ADIZ; I EXPECTED A HDOF TO POTOMAC APCH. AS I REACHED THE ADIZ BOUNDARY; I QUERIED PATUXENT APCH; REQUESTING A HDOF. I WAS TOLD 'UNABLE TO OBTAIN A HDOF; RADAR SVCS TERMINATED; SQUAWK 1200; FREQ CHANGE APPROVED.' I SWITCHED SQUAWK AND IMMEDIATELY CONTACTED POTOMAC APCH ON 124.55. THE CTLR RESPONDED TO MY CALL WITH MY CALL SIGN AND 'STAND BY.' MY ERROR WAS ASSUMING THAT ONCE CONTACT HAD BEEN ESTABLISHED AND A FLT PLAN WAS ON FILE; I COULD CONTINUE TO PROCEED N ON THE ADIZ BOUNDARY. WHEN THE CTLR FINALLY RESPONDED; I WAS INFORMED I WAS INSIDE THE ADIZ WITHOUT AUTH. ALSO; I WAS CONFUSED AND THOUGHT THE ORIGINAL SQUAWK GIVEN BY PATUXENT APCH WAS MY ADIZ SQUAWK.