Today marks the firing of the starting gun in the next phase of IPv4 exhaustion.  To date, running NATs on the ISP side has generally been the preserve of the mobile operators, where running servers generally isn’t something you’d want or need to do over a mobile connection.  However, something new happened today – Plusnet, the Sheffield-based (and BT Group owned) ISP announced they are going to run a 3-week trial of CGNAT, or Carrier Grade NAT.

Some more info on that can be found here:

http://www.ispreview.co.uk/index.php/2013/01/isp-plusnet-trials-controversial-ipv4-address-sharing-as-ipv6-alternative.html
http://www.thinkbroadband.com/news/5658-plusnet-in-trial-of-carrier-grade-nat-to-conserve-ipv4-addresses.html
http://www.theregister.co.uk/2013/01/15/ipv4_nat/

Now, the term “carrier grade” has, to date, normally meant something along the lines of big and impressive, and can generally handle thousands or millions of whatever it can do, and generally you’d expect a “carrier grade” product to be somehow better than a product which is “non-carrier grade”.  However, this is probably the one “carrier grade” product that generally makes things worse.  A bit of explanation here – “carrier grade” in this context means that it is suitable for a “carrier”, or telco, to use.  CGNAT is “carrier grade” in the sense that it can handle thousands and thousands of connections at once, and runs on slightly beefier hardware than your average home ADSL router, but the net effect of running CGNAT actually makes the experience worse than it would otherwise have been, since you are now removing the ability for that ISP’s users to do port forwarding through the ISP’s NAT which will almost certainly stop quite a lot of things working, for example anything that requires an incoming port (e.g. a server), or perhaps things like UPnP and even Skype.  This is probably going to cause quite a lot of compaints, depending on how much it breaks, and also means that users will be getting a consderably degraded Internet experience than they already are (since NAT is not exactly how IPv6 was intended to be used in the first place).

Plusnet had an IPv6 trial, which they stopped for some unexplained reason, and so far no word on when or if it will be resurrected.  Rolling out CGNAT and not also rolling out IPv6 seems very short-sighted to me, but more importantly the fact that they are even considering CGNAT at all suggests to me that there is one AS in the UK that could be running dangerously low on its allocated IPv4 addresses (with no prospect of obtaining any more from RIPE).

I shall be watching this space with interest…