Rostov on Don vs Rostov na Donu

Rostov on Don and Rostov na Donu both are nouns.

Word	Noun	Adjective	Verb	Adverb
Rostov on Don	Yes	No	No	No
Rostov na Donu	Yes	No	No	No

As nouns, Rostov on Don and Rostov na Donu are synonyms defined as:

Rostov on Don and Rostov na Donu: a seaport on the Don River near the Sea of Azov in the European part of Russia

Other synonyms of Rostov on Don include Rostov-on-Don, Rostov.

Rostov on Don (noun)	Rostov na Donu (noun)
a seaport on the Don River near the Sea of Azov in the European part of Russia	a seaport on the Don River near the Sea of Azov in the European part of Russia