Itahari in the context of Sunsari District

Koshi Province (Nepali: कोशी प्रदेश, romanized: Kōśī pradēśa) is an autonomous province of Nepal adopted by the Constitution of Nepal on 20 September 2015. It covers an area of 25,905 km (10,002 sq mi), about 17.5% of the country's total area. With the industrial city of Biratnagar as its capital, the province includes the towns of Birtamod, Sundar Haraincha, Damak, Dharan, Itahari, Triyuga Municipality and Mechinagar, and Mount Everest, Kangchenjunga and Ama Dablam. Koshi River, the largest river of the nation, forms the province's western boundary. Under the First-past-the-post voting system issued by the Constituency Delimitation Commission, Nepal, the province hosts 28 parliamentary seats and 56 provincial assembly seats.The province is bordered by the Tibet Autonomous Region of China to the north, the Indian states of Sikkim and West Bengal to the east, Bihar to the south, and Bagmati Province and Madhesh Province to the west. According to the 2021 Nepal census, there are around five million people in the province, with a population density of 190 per square kilometre. In the 2011 Nepal census, the province had approximately 4.5 million people.

Biratnagar (Nepali: विराटनगर) is a metropolitan city in Nepal, which serves as the capital of Koshi Province. With a Metropolitan Urban Agglomeration population of 244,750 as per the 2021 census, it is the largest city and fastest growing city in the province and also the headquarters of Morang district. As per the preliminary report of 2022 Nepal census, Biratnagar Metropolitan has an estimated city population of 243,927 excluding the suburban areas which are now very well connected to the old town. It is the administrative center of the Greater Birat Development Area which incorporates the cities of Biratnagar and its suburbs and towns of-Itahari-Gothgau-Biratchowk-Dharan primarily located on the Koshi Highway in Eastern Nepal, with an estimated total urban agglomerated population of 804,300 people in 159,332 households including the town of Ithari. Biratnagar is located 399 km (248 mi) east of the capital, Kathmandu, and 6 km (3.7 mi) north of the bordering town of Jogbani in the Indian state of Bihar which is very well connected through Rani Path (old highway) and Koshi Path (new highway), and serves as a major entry point from India.

Biratnagar was declared a metropolitan city on 22 May 2017, a merger with additional wards pushing the total population to over 240,000. It is the sixth largest city of Nepal, with 244,750 inhabitants as per 2021 Nepal census.

Sundar Haraicha (Nepali: सुन्दर हरैंचा) is a municipality in Morang District of Koshi Province in Nepal. It is centered around Biratchowk town which is the gateway to Biratnagar. It borders Itahari sub-metropolitan city to the west, Dharan sub-metropolitan city to the northwest and Belbari municipality to the east and is connected by Mahendra Highway. It is one of the fastest growing cities in eastern Nepal and the second largest city of Morang district after Biratnagar, with 121,305 inhabitants living in 26,677 households. It includes many smaller towns, including Salakpur, Biratchowck, Khorsane, Gachhiya, Haraicha, Dulari, and Baliya. It consists of the town centres of Biratchowk and Gothgaun which are parts of the Greater Birat Development Area which incorporates the cities of Biratnagar-Itahari-Gothgau-Biratchowk-Dharan primarily located on the Koshi Highway in Eastern Nepal, with an estimated total urban agglomerated population of 804,300 people living in 159,332 households.

It was established on 10 March 2017 by merging the former municipalities Sundar Dulari(Sundarpur VDC and Dulari VDC) and Koshi Haraicha(Indrapur VDC, Mirgulia VDC and Haraicha VDC). At the time of the 2011 Nepal census, the localities formed had a joint population of 80,518 people living in 18,610 individual households.

Dharan (Nepali: धरान) (Limbu: ᤎᤠᤖᤠᤴ, Dhaaraan; ᤒᤡᤈᤧᤐᤢᤷ ᤜᤱᤐᤱ ᤐᤱᤕ᤺ᤰ, Beejaepoor Hangpang Pangya:k: in Limbu) is a sub-metropolitan city in Sunsari District of Koshi Province, in eastern Nepal, which was established as a fourth municipality in the Kingdom in 1958. It is the third most populous city in eastern Nepal after Biratnagar and Itahari. The Nepali word "dharan" means a saw pit.

In 1960, a British Gurkha camp was also established near the city. The use of the camp by British Gurkhas finished in the mid-1990s. Dharan has an estimated city population of 173,096 living in 34,834 households as per the 2021 Nepal census. It is one of the cities of the Greater Birat Development Area which incorporates the cities of Biratnagar-Itahari-Gothgau-Biratchowk-Dharan primarily located on Koshi Highway in eastern Nepal, with an estimated total urban agglomerated population of 804,300 people living in 159,332 households. It is the largest city in the Koshi Province by area, at 192.61 square kilometres, and Biratnagar and Itahari are the second- and third-largest cities by area respectively.