* First time almost entire country has been sealed off
* Small number of ISPs makes shutdown easy
* Facebook and Twitter particularly vulnerable
By Georgina Prodhan
LONDON, Jan 28 (Reuters) - The move by Egyptian authorities to seal off the country almost entirely from the Internet shows how easily a state can isolate its people when telecoms providers are few and compliant.
In an attempt to stop the frenzied online spread of dissent against President Hosni Mubarak's 30-year rule, not only Facebook and Twitter but the entire Internet was shut down overnight, leaving some 20 million users stranded.
Hundreds of service providers offer connections in Egypt, but just four own the infrastructure -- Link Egypt, Vodafone/Raya, Telecom Egypt and Etisalat Misr.
Daniel Karrenberg, chief scientist at RIPE NCC, a European not-for-profit Internet infrastructure forum, says immature markets with few providers can achieve such shutdowns relatively easily.
"The more simple the topology is and the fewer Internet services providers there are, the easier it is for any government or the telco themselves to control access into any geographical area," he said.
"If you have a relatively diverse telecoms market and a very much meshed Internet topology then it's much more difficult to do than if you have the traditional telecoms structure of two decades ago and they control all the international connections.
"Obviously that creates a choke point," he said.
Despite the rapid transformation of the Web during its short history, and the unprecedented freedom of expression it has enabled, the Internet still has vulnerable points that can be exploited by governments or for commercial interests.
"Virtually all of Egypt's Internet addresses are now unreachable, worldwide," Jim Cowie, chief technology officer of U.S.-based Internet monitoring firm Renesys wrote on the company blog (www.renesys.com/blog).
"Every Egyptian provider, every business, bank, Internet cafe, website, school, embassy, and government office that relied on the big four Egyptian ISPs for their Internet connectivity is now cut off from the rest of the world."
Vodafone said in an emailed statement: "All mobile operators in Egypt have been instructed to suspend services in selected areas. Under Egyptian legislation, the authorities have the right to issue such an order and we are obliged to comply."
A few large organisations with independent connections were able to stay connected to the Internet.
Cowie said on Friday he was investigating two apparent exceptions to the block: the Commercial International Bank of Egypt and the Stock Exchange.
Iran, Tunisia and most recently Syria have imposed Internet restrictions in attempts to quell opposition, but Egypt's is by far the most drastic move so far.
The closest precedent has been in China, which has more Internet users than any other country and also the strictest controls. It cut off Internet access to its Xinjiang region for almost a year after deadly ethnic unrest in 2009.
The world's biggest social network Facebook, and Twitter with its real-time mini-blog posts, have proved extraordinarily effective in gathering large numbers of people together and helping them to be nimble in dodging the authorities.
Lynn St Amour, president of the Internet Society, says they could have made revolutionaries of many who had not seen themselves as activists, thanks to the ease of signing up to groups or sending messages of support while sitting at home.
But the danger of depending on such services is that they can be blocked simply by targeting their IP addresses, since they are centralised on a single site -- as witnessed in Iran and Tunisia.
"It's quite easy, as we've seen," St Amour told Reuters at the World Economic Forum in Davos.
In Tunisia, dissidents even found their Facebook pages taken over without their knowledge -- something that Facebook was able to resolve because its own software had been hacked.
But when access to an entire site is blocked from outside, there is little that Facebook or Twitter can do -- although users often find ways around the problem by using proxy servers.
"We try very hard to keep Facebook available wherever people want to access it," Dan Rose, who is responsible for Facebook's worldwide business development, said in London this week.
"We have outreach and relationships with governments all around the world. "We can only do what we can do."
The resilience of the Internet in any particular country also depends on the diversity of its international providers, the routes in an out of a country.
In 2008, Egypt suffered an 80 percent outage of Internet services when submarine cables in the Mediterranean linking Egypt to the rest of the world were accidentally cut.
On Friday, key fibre-optic cables that pass through Egypt as they link Europe to Asia appeared unaffected.
Renesys's Cowie contrasted a country such as Egypt with those that have highly dispersed international connections.
"In the United States you have every global carrier available to you, you have multiple cable landing points ... you have a country that effectively can't be taken off the Internet," he told Reuters. (Additional reporting by Kenneth Li in Davos; editing by Sitaraman Shankar and Andrew Roche)