Original URL: http://www.theregister.co.uk/2013/01/15/ipv4_nat/
Sheffield ISP: You don't need a whole IPv4 address to yourself, right?
PlusNet seeks volunteers - just for a tick, until IPv6
Facing the shortage of IPv4 addresses, and glacial adoption of IPv6, UK ISP PlusNet is looking for volunteers among its customers to test out sharing the IPv4 addresses on its network.
The technique being tested by PlusNet uses a NAT (Network Address Translator) to share a single internet-facing IP address between multiple customers, eking out the limited supply of addresses - but at the cost of denying incoming connections. That will screw up some users, which is why PlusNet is looking for volunteers, as it may prove acceptable to many.
Mobile operators have been doing this for years: millions of customers on a single network typically share three or four IP addresses between them, reducing the need for IPv4 addresses and (importantly) making it impossible to create connections from the internet to a mobile phone - until it hops onto a Wi-Fi network.
But fixed providers have always given individual addresses to each customer, generally allocated at connection and retained for at least the duration of that connection. But these days, there aren't enough addresses to go around, which is prompting more bodging while we continue to wait for IPv6.
Most local networks already use a NAT, sharing a single external IP address with multiple devices in the home or office. One's NAT will face the internet with a publicly reachable IPv4 address of, say, 188.8.131.52, which is owned by one's ISP. The NAT allocates private IP addresses to devices on the network, often starting 192.168.x.x as those numbers are reserved for this purpose, and then maps outgoing connections so that returned data (from websites and such) is routed to the requesting computer.
As well as eking out the supply of addresses, the NAT makes it impossible for anyone on the internet to establish a connection to a computer behind it, as those computers don't have valid IPv4 addresses, so anyone hosting a web server on their own network has to configure the NAT with some "port forwarding" so incoming connections arriving at the NAT are sent to the right place.
But if the NAT is owned by PlusNet, then port forwarding isn't possible, and one can't host services on a home network just as one can't run host a web server in a mobile network today.
The days when everyone ran their own servers are long gone and almost everything is available in the (better secured) cloud these days, but PlusNet wants to know what other apps might be affected (Tor, BitTorrent and other P2P stuff being areas of concern) so is asking for volunteers  interested in having a stab at sharing an IP.
But NATs don't just have technical implications, they also make it very hard to see who's doing what. ISPs log dynamically allocated IPs for legal redress, and PlusNet would do that with those behind the NAT too. All of this is fine when the police are involved but here at El Reg we count IP addresses as a rough guide to readership, and numerous sites including Wikipedia block unwanted edits by IP address, so sharing an IP could mean sharing responsibility too.
The solution is for everyone to switch to IP version 6 (odd versions are experimental, so we're currently on version 4 - which is, technically, the second). IPv6 has much longer addresses and won't end up clumped with hoarding telcos like IPv4 did, but it means upgrading every router and device on the internet, not to mention convincing all the geeks that it's necessary, if they're not to end up drugged into submission - which was recently the terrible fate of our own Verity Stob . ®