DISPATCHER DISCOVERS ACFT SENT ACROSS GULF WAS NOT OVERWATER EQUIPPED.

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I WAS DISPATCHER HANDLING MEXICO CITY ARRS. RELEASED SCHEDULED FLT MIA TO MEX IN MLG EQUIP. NOTHING ABOUT RELEASE STOOD OUT -- WAS NORMAL; ROUTINE. FLT WAS RELEASED ACROSS GULF AND FLT WAS FLOWN WITH NO APPARENT PROBLEMS. UNFORTUNATELY; NO ONE NOTICED UNTIL AFTER THE FLT THAT IT WAS NOT AN OVERWATER EQUIPPED MLG. DOMESTIC ACFT RTED ON FLT IN ERROR; I BELIEVE. IF RTED INTENTIONALLY BY RTERS; NO NOTICE WAS GIVEN. ONLY WAY TO TELL IT WAS WRONG MLG WAS TO NOTICE PLANE NUMBER IN FLT PLANNING FORMAT. NORMALLY; A 19XX SERIES MLG THIS TIME WAS A 14XX SERIES MLG. THIS ROUTING OF DOMESTIC MLG HAS HAPPENED BEFORE AND I'VE CAUGHT IT; BUT I MISSED IT THIS TIME. THE FLT PLANNING SYS IS ENTIRELY COMPUTERIZED BUT UNFORTUNATELY; CAN'T DIFFERENTIATE BTWN DOMESTIC AND OVERWATER ACFT. WE USE INTL FLT PLAN FORMAT FOR MEXICO FLTS BUT NO COMPUTER PROGRAM TO AVOID THIS HAPPENING. A COMPUTER SOPHISTICATED ENOUGH TO DO ALL OUR FLT PLANNING SHOULD BE ABLE TO BE PROGRAMMED TO AVOID THIS. THIS WAS SUPPOSED TO BE A DAY OFF; BUT I VOLUNTEERED TO WORK THIS MIDNIGHT SHIFT FOR FUTURE TIME OFF. MAYBE BECAUSE IT WAS A SINGLE MID-SHIFT IN THE MIDDLE OF MY DAYS OFF; AND MAYBE BECAUSE I WAS UP ALL DAY AND ONLY HAD A COUPLE OF HRS OF SLEEP BEFORE GOING IN; MAYBE AT AM IN THE MORNING I WASN'T AS ALERT AS 'NORMAL.' I'LL BE WATCHING THOSE ACFT NUMBERS MORE CLOSELY.