Previous generations of core mobile network, would only allocate a single IP address per UE (Well, two if dual-stack IPv4/IPv6 if you want to be technical). But one of the cool features in 5GC is the support for Framed Routing natively.
You could do this on several EPC platforms on LTE, but it’s support was always a bit shoe-horned in, and the UE was not informed of the framed addresses.
If you’ve worked in a wireline ISP you’re probably familiar with the concept of framed routing already, in short it’s one or more static routes, typically returned from a AAA server (Normally RADIUS) that are then routed to the subscriber.
Each subscriber gets allocated an IP by the network, but other IPs can also be routed to the subscriber, based on the network and CIDR mask.
So let’s say we allocate a public IP of 188.8.131.52/32 to our subscriber, but our subscriber is a fixed-wireless user running a business and they want a extra public IP Addresses.
How do we do this? With Framed Routing.
Now in our UDM we can add a “Framed IP”, and when the SMF sets up a session for our subscriber, the extra networks specified in the framed routes will get routed to that UE.
If we add 184.108.40.206/30 in our UDM for a subscriber, when the subscriber attaches the UPF will be setup to forward traffic to 220.127.116.11/32 and also traffic to 18.104.22.168/30 to the UE.
Update: I previously claimed:
Best of all this is signaled to the UE during the attach, so the UE is say a router, it becomes aware of the Framed IPs allocated to it.
This is incorrect! Thanks to Anonymous Telco Engineer from an Anonymous Nordic Country for pointing this out, it is not signaled to the UE.
More info in 3GPP TS 23.501 section 5.6.14 Support of Framed Routing.