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UPD ISPs React to the Dangers of Adopting IPv4 Internet Address Sharing

Tuesday, Jan 22nd, 2013 (9:15 am) - Score 10,781

Internet providers from around the United Kingdom have given a decidedly mixed response after we asked a group of them whether IPv4 address sharing (Carrier Grade NAT) was the future of fixed line broadband (i.e. until full IPv6 adoption). CGNAT allows a single IP address to be shared between several users but this can introduce serious problems.

So why should you care? Everybody needs an Internet Protocol (IP) address to go online and your ISP usually assigns one to your connection (it’s the internet equivalent of a phone number). At present we call these IPv4 (e.g. addresses but those are running out and its replacement, IPv6 (e.g. 2001:cdba::2257:9652), is not directly compatible with the old standard (UK ISPs Explain IPv6 Readiness Fears).

Most ISPs can get around this problem by installing a dual-stack network, which allows IPv4 and IPv6 addresses to communicate, but it will take some years before IPv6 is completely ready to take-over. In the meantime IPv4 addresses will still be needed and are likely to become an increasingly rare commodity.

The internet’s phone number system is essentially being split between two standards and in order to continue adding new connections some ISPs will need to ration their remaining IPv4’s and or share one address between several users (CGNAT). The situation arose because the wider internet industry (modem makers, software developers and big ISPs alike etc.) effectively left it until the last minute to adapt.

What’s the problem with CGNAT?

The problem is that a large number of advanced internet services and configurations don’t work well when IP’s are shared and likewise there are some serious security considerations. The situation partly arises because most of these services assume that everybody will have a unique address (the end-to-end principal), which is broken by CGNAT.

Axel Pawlik, MD of RIPE NCC (EU Internet Registry), told ISPreview.co.uk:

Carrier Grade NAT (CGNAT) has a number of limitations which need to be considered. Most importantly, it goes against the open spirit of the Internet and the principles which have generated such incredible innovation over the years. There is also a potential for compromised functionality for the end user – relying on just one IP address that is then split into many can be an issue because if there is a problem it will impact many people, instead of just one.

It is good that [PlusNet are running a trial] because that will give everyone a chance to evaluate the wider implications and whether there is any long term benefit to CGNAT versus speeding up the deployment of IPv6.”

Thankfully most home users just need the internet for basic web browsing, video streaming and email services, which should be fine (note: some video services like Netflix may still have problems). Sadly the millions who also enjoy multiplayer gaming (e.g. XBox Live, PlayStation Network or game server hosting), use legitimate P2P based services, require the ability to forward ports on their broadband router, host FTPs and web servers (hosting any online server would be extremely difficult) could experience connection or performance problems.

On top of that we have the security considerations, which are numerous. For example, if somebody else with your now “shared” IP is banned from sending email or accessing a particular website then you too could be affected by the same block. Likewise some bank accounts need a unique IP for access and having shared users would be a potential risk and could, in extreme cases, result in you being blocked entirely.

CGNAT also makes it harder and more expensive, but not impossible, to keep accurate track of user activity for law-enforcement purposes. So, generally speaking, few ISPs would consciously choose to do CGNAT unless it was truly necessary but that’s exactly the situation we now find ourselves approaching.

It’s similarly worth remembering that some of the services most likely to be affected are also those used by many of the online world’s most vocal communities (e.g. gamers and IT folk). ISPs that fail to show respect for this could risk a bruising backlash and many businesses could suffer if their related services are no longer usable.

What do ISPs say?

Some ISPs believe that they have enough spare IPv4 addresses to avoid ever needing to go down the path towards CGNAT, although most appear to be far less certain.

PlusNet kicked things off last week when it revealed plans to conduct a three week trial of the solution, which drew a lot of attention because until recently many people had assumed that the ISPs parent, BT, would have enough spare IPv4 addresses to last for several years. But PlusNet should be commended for being brave enough to launch the first trial and thus open this debate.

Please head to Page 2 for more ISP responses and our conclusion..

By Mark Jackson
Mark is a professional technology writer, IT consultant and computer engineer from Dorset (England), he also founded ISPreview in 1999 and enjoys analysing the latest telecoms and broadband developments. Find me on X (Twitter), Mastodon, Facebook and .
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