Kocaeli is a province of Turkey and one of only two to not have the same official name as its capital. This is Izmit, which is thus also sometimes called Kocaeli. The province is the successor of the Ottoman-era Sanjak of Kocaeli. The largest towns in the province are Izmit and Gebze. The traffic code is 41. The province is located at the easternmost end of the Marmara Sea around the Gulf of Izmit. Kocaeli is bordered by the province of Istanbul and the Marmara Sea to the west, the Black Sea to the north, the province of Sakarya to the east, the province of Bursa to the south and the province of Yalova to the southwest. The metropolitan area of Istanbul extends to the Kocaeli-Istanbul provincial border. The size and natural conditions of the Bay of Izmit allow for extensive port facilities, including the Golcuk Naval Base. The province is called the industrial capital of Turkey.