I have been reading about CGN. Though similar to NAT it is used by service providers to route traffic without the need for the a public IP address. But when the packet leaves the ISP to the public internet, it does need a public IP address. So how exactly does CGN help save IP addresses. Or what are the other advantages of CGN.

For without CGN (but NAT + PAT) the packet cycle is:

customer device -> router (NAT - private to public) -> public internet -> server (maybe a DNAT here)

with CGN this changes to

customer device -> router (NAT - private to public) -> ISP router (CGN here) -> packet travels through the ISP -> NAT (CGN address -> public ) -> public internet -> server

So how is having an additional layer of NATing helping the case ?

  • From my understanding, an extra layer of NAT anywhere will save addresses. It sort of sounds to me like a private network within a private network.
    – Duffman
    Feb 5, 2018 at 21:48

2 Answers 2


The RIRs have run out of public IPv4 addresses to assign to the ISPs, so the ISPs are hoarding their precious pools of public IPv4 addresses for business customers. To do this, they are rolling out CGN to residential customers. This allows the ISPs to use a single public IPv4 address for thousands, or tens of thousands of residential customers.

The downside of this is that the residential customers can no longer violate their terms of service and run services to the Internet. The ISPs really don't care because it is a violation of the residential terms of service, and the customers will need to get (more expensive) business service and public IPv4 addresses (as they were supposed to, anyway).

  • So it is like increasing the domain within which the private addresses are used ? Instead of doing it for a few residential customers in a given area. Thousands are accumilated and a single IP pool is used to serve all of them ? Also doesn't NAT + PAT achieve the same purpose of saving IP addresses ? what additinal is CGN providing here ?
    – john
    Feb 5, 2018 at 21:52
  • 1
    CGN should really use the Shared address space, rather than Private address space, although some ISPs use Private address space, which causes problems for some of their customers. CGN is NAPT, but instead of each customer using a valuable public IPv4 address, many customers will share a single public IPv4 address. That can save many thousands of public IPv4 addresses for the business customers.
    – Ron Maupin
    Feb 5, 2018 at 21:59
  • If you need to serve a few thousand customers and only have got a /24 address range you set up CGN and put the /24 addresses into the NAT pool.
    – Zac67
    Feb 5, 2018 at 21:59
  • Sharing one IP among tens of thousands of customers is ridiculous, so is your implication that the only people hurt by this are those violating their terms of service. Feb 5, 2018 at 22:27

Or what are the other advantages of CGN.

Im quite sure: None.

As far as I know CGNAT is only used by internet providers who do not have enough IPv4 addresses to give every customer an own IPv4 address and therefore must use CGNAT.

... and there are even providers (not having enough IPv4 addresses) where you have to pay extra money for not getting CGNAT!

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