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How can the Internet have too many routes and not enough addresses?

Stephen Lawson | Aug. 21, 2014
As IPv4 runs out of space, attempts to get around the problem could make things even more complicated.

The depletion of Internet addresses would seem to spell relief for aged routers that are struggling to deal with the Internet's growth, but the complicated interplay between those trends might cause even more problems.

Last Wednesday, some older routers and switches stumbled when the Internet's table of routes surpassed 512,000 entries, the maximum they could hold in a special form of memory called TCAM (Ternary Content Addressable Memory). The event drew widespread attention, though it was actually the third time in this young century that the Internet had broken through such a threshold. The number of routes exceeded 128,000 around 2003 and 256,000 in 2008, each time causing problems for some outmoded gear.

Devices that don't have room for all the routes may reboot themselves or fail to route some traffic, but the affected gear was fairly old. Cisco Systems says all the routing products it's sold for at least the past two years have had enough room in TCAM for more than 512,000 routes. Routers designed for the cores of carrier networks surpassed that long before. Juniper Networks, Cisco's longtime router rival, said it updated its gear for this problem more than 10 years ago. Alcatel-Lucent said its routers use a different memory architecture from the devices that got hit with the problem.

Because almost all the addresses defined by IPv4 (Internet Protocol version 4) have already been handed out to Internet service providers or end users, the number of routes allocated under that system may not grow much more, according to Cisco engineers. That would be one silver lining on a cloud that's hung over the network of networks for years.

"IPv4 cannot grow forever. We already reached a certain limit, so we personally wouldn't expect it to grow much larger," said Sasa Rasovic, incident manager at Cisco's Product Security Incident Response Team.

However, another danger remains, and it comes from the address depletion itself. With fewer IPv4 addresses at hand, users or service providers may want to split them up into smaller routes.

By common agreement among Internet engineers, the smallest accepted route on the Internet today points to a block of 256 consecutive IP addresses. (Using private addresses, companies and service providers can hook up many more devices behind those globally unique ones.) Now, some network operators want to break up those blocks so they can satisfy more customers, said Jim Cowie, chief scientist at Dyn, a traffic management company that recently acquired Internet analysis firm Renesys. Then, instead of one Internet route to reach the 256 addresses, there would be two.

"People are trying to do more with less," Cowie said.

Along the way, some may also be putting profit ahead of the Internet's ease of use. IP addresses officially are handed out free by nonprofit regional authorities, but their supplies are mostly gone. The mad dash for IPv4 addresses has led to some unseemly practices by those who already got their addresses.


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