The particular term was used in the context of diplomacy by the western states, as their diplomats were received at "porte" (meaning gate). During the constitution period (see Young Turk Revolution) the functions of the Divan were replaced by the imperial government, and "porte" came to refer to the Foreign Ministry. During this period Grand vizier came to refer to the position of a president and viziers became the Ottoman Senate.
The Sublime Porte was the name of the open court of the sultan, led by the Grand Vizier. It got its name from the gate to the headquarters of the Grand Vizier in Topkapı Palace in Istanbul, where the sultan held the greeting ceremony for foreign ambassadors. It was an ancient Oriental practice to make the gates of cities and kings' palaces places of assembly.
Later the name came to refer to the Foreign Ministry and in contemporary times the office of the governor (Vali) of Istanbul Province. This name has also been interpreted as referring to the Empire's position as gateway between Europe and Asia.
The High Porte, in contrast, referred to the private court of the sultan. Porte is French for "gate"; therefore, the term High Porte is a bilingual combination of English High and French Porte that is equivalent to Bab-ı Ali.
The Sublime Portal is also the name of a website for expats in Turkey. The website includes a discussion forum and also a wiki and can be found here