CGNAT - Seeking Real World Experience

Adam adamkuj at
Wed Nov 23 21:17:01 UTC 2016

I'm crunching the numbers on the cost effectiveness of implementing CGN vs
IPv4 auctions. The determining factor is how many ephemeral ports are
reserved for each customer. This is for a residential broadband environment.

Is anybody doing deterministic NAT/PAT (i.e. each customer gets X ports -
no more, no less)? My thinking is that X=8192 would cover even the power
users. However, with only 8 customers per public IPv4 address, CGN is not
worth the trouble. With X=8192, our money and time would better be spent
acquiring additional IPv4 space. Are people successfully using a smaller
deterministic port allocation? What's your X?

If I can't make the numbers work for deterministic NAT, I might be able to
live with dynamic port assignments. Specifically, I'm referring to what
vendor J calls "Port Block Allocation". I was thinking 1024 ports per
block, with up to 8 blocks per customer (and a bunch of log correlation to
determine who was using which ip:port tuple at a given datetime). I *can*
make the math work out in favor of CGN if the average customer uses <= 3072
ports (3 blocks). But is that going to be enough? I'd love to hear other
people's experiences.


More information about the NANOG mailing list