£30 for a Gigabit of Fibre, There's a Reason It's So Cheap

You may have seen adverts promising extremely fast full-fibre broadband at surprisingly low prices. In some parts of the UK, alternative network providers offer gigabit-capable connections for around £30 per month, usually substantially less than comparable packages from the more well-known internet service providers.

The Catch: CG NAT

These fibre services utilize a technology called CG NAT to save money. While this is not an issue for browsing the internet and watching Netflix at home, for business use it causes problems for certain types of internet connected services you may have including:

• VoIP phone services
• Self-hosted websites and file sharing
• Video streaming from security systems

With VoIP phones, the problems can manifest themselves in symptoms similar to those of SIP ALG, where outbound calls work but inbound calls do not, or you may get one-way audio.

What is CG NAT?

An IP address uniquely identifies your network devices on the internet. CG NAT or Carrier Grade Network Address Translation, is a system by which multiple customers of an internet service provider share the same IP address.

When you request a webpage or start a video stream, the connection begins from inside your network. The provider can record which customer started the connection and direct the response back to the correct router. This is why normal browsing, streaming and downloading usually work without difficulty.

An incoming connection, such as a phone call, is different. If somebody on the internet tries to connect to a port on the shared public address, the provider's CGNAT equipment may have no way to determine which customer should receive it.

Can I fix this with a setting on my router?

No, configuration changes on your own router cannot solve this because the connection is being blocked or translated by your ISP before it reaches your router.

What to do?

If you are considering switching to a cheap fibre provider, be aware of issues that CG NAT can cause.

Before signing a contract, ask the provider the following question:

Does this package include a publicly routable IPv4 address, or is the connection behind CGNAT?

Often, they can offer you a service without CGNAT, but for an additional monthly fee. If you are already in a contract you can often call them up and ask them to remove CGNAT, they will handle this as a package upgrade and increase your monthly bill.

Updated on 31/07/2026 by David Wood

As senior cloud solutions architect at TelephoneSystems.cloud, with a degree in computing, I bring over 29 years of extensive experience in cloud technology.