"As the IPv4 address space is now depleted, a few smaller routes ... are being sold to other entities. Apart from a number of other more serious issues this is causing to the Internet community at large, this also has potential to cause a growth of the routing table size," Cisco's Rasovic said in an email message. "It's hard to predict just how fast and how big of an impact this will have in the future."
If some service providers start to split up the smallest blocks into even smaller ones, that could even affect whether all users can reach everyone else on the Internet, Dyn's Cowie said. Other operators might filter out the smaller routes, keeping their own routing tables a more reasonable size but not offering access to some addresses, he said.
And though it's impossible to say how many new routes might result, routers would continue to face a growing number of them. Like new party guests who want a piece of the same pie, Internet address holders could cut the IPv4 address space into ever smaller pieces, and it would fall onto the routers to keep track of all the slivers.
Dave Schaeffer, CEO of ISP Cogent Communications, thinks the routing tables will keep growing just from new addresses coming online.
"There's still a big, dark pool out there of IPv4 addresses in the hands of service providers that can be routed, that are not routed (yet)," Schaeffer said.
Migrating to IPv6 would eliminate the address shortage, because the newer protocol has an almost unlimited supply. Few users have adopted IPv6 even though most systems and networking gear have long been equipped for it. The IPv6 routing table still only has about 20,000 routes in it, Cowie said. That's what makes it feasible for Cisco to suggest, among other things, that network operators reassign some of the memory in their routers that was automatically set aside for IPv6 routes and give it to IPv4 routes.
But the short supply of older addresses and the expected growth of the Internet of Things eventually will bring more IPv6 addresses into service, Cowie said. That will raise issues of its own.
"Now that IPv6 has been introduced, more and more devices are going to be connected," Rasovic said. "The tables are different [in IPv6], and they're managed differently in memory." It's hard to know how many IPv6 routes there could eventually be, Cowie said. Those routes will all take up more memory, because an IPv6 address is much longer than one from the older version. Network engineering groups are already trying to figure out how to manage IPv6 routes, according to Cisco.
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