Londonderry is the best city in the UK for high-speed broadband availability, according to the national regulator.
But some customers in poorer parts of Belfast are not getting their internet access at the same speed as people in the richest areas, Ofcom also found.
Superfast broadband coverage varies widely between major urban areas in the UK, but Derry is the best-performing city for availability at 99%.
While almost everyone in Northern Ireland's largest cities have access to superfast broadband, in Glasgow one in three people does not.
As part of the study, Ofcom also commissioned case studies in six UK cities, including Belfast, to explore how the use and availability of broadband in urban areas varied according to social deprivation. It found that there were more slower broadband connections in poorer areas of Belfast.
Although almost all parts of the city had access to superfast broadband, around 5.9% of connections in the poorest areas had speeds below 2 Mbit/s (megabits per second).
But this fell to just 2.2% in areas with the highest incomes.
Ofcom said the results suggested that, in many cases, low incomes may be a barrier to customers choosing services with higher speeds.
Ofcom's consumer group director Claudio Pollack said: "Access to fast broadband is an important part of modern life, and a source of economic growth and investment across the UK.
"We know from previous research that rural areas often lack fast broadband coverage, something the Government is helping to address with public funding.
"Today's findings suggest that the usage and availability of faster broadband vary widely between cities. We will carry out further work in this area to help bring faster broadband to UK homes."
STORY SO FAR
Some internet users living in urban areas are still putting up with "very low" broadband speeds, with disadvantaged areas suffering the slowest services, an Ofcom study has found.
The study examined 11 UK cities and found wide variations in the proportion of premises on a relatively slow broadband connection.
Access to superfast broadband in Derry is the best in the UK.