NDHS was built in 1922, when Dallas ended at Mockingbird. Present-day North Dallas is generally considered the area North of NW Hwy, which wasn't built until the '30s. Many people include suburban cities. In about 10 years, that will extend to Texoma.
Next stop on the Urban Sprawl, OKC.
The various answers here explain/demonstrate why there is confusion...
Keep in mind, there is a difference which some posters fail to make when referencing the location they have in mind -- between "North Dallas" and "FAR North Dallas". That latter, might be deemed part of "North Dallas" --- which would be such a large area as to not be too helpful in locating the proximity of a business (or a provider). "FAR North Dallas" has as its general boundaries : the DNT - GBTP - Coit - I635 -- and proximate neighborhoods which fall within the municipal borders of the city of Dallas.
Basically, the 635 marks the boundary between "North" and "Far North" Dallas.
Can I see a show of hands of those born in Big D? Right here baby...I grew up off of Forest Lane and Midway (when Forest was a drag strip...which nowadays means something entirely different) and North of 635 was the Frontier