Large Scale NAT represents the most common deployment of S-NAPT in carrier networks today, it is already employed by mobile operators around the world for handset access to the Internet.
A Large Scale NAT is typically deployed in a central network location with two interfaces, the inside toward the customers, and the outside toward the Internet. A Large Scale NAT functions as an IP router and is located between two routed network segments (the ISP network and the Internet).
Traffic can be sent to the Large Scale NAT function on the 7750 SR using IP filters (ACL) applied to SAPs or by installing static routes with a next-hop of the NAT application. These two methods allow for increased flexibility in deploying the Large Scale NAT, especially those environments where IP MPLS VPN are being used in which case the NAT function can be deployed on a single PE and perform NAT for any number of other PE by simply exporting the default route.
The 7750 SR NAT implementation supports NAT in the base routing instance and VPRN, and through NAT traffic may originate in one VPRN (the inside) and leave through another VPRN or the base routing instance (the outside). This technique can be employed to provide customers of IP MPLS VPN with Internet access by introducing a default static route in the customer VPRN, and NATing it into the Internet routing instance.
As Large Scale NAT is deployed between two routed segments, the IP addresses allocated to hosts on the inside must be unique to each host within the VPRN. While RFC1918 private addresses have typically been used for this in enterprise or mobile environments, challenges can occur in fixed residential environments where a subscriber has existing S-NAPT in their residential gateway. In these cases the RFC 1918 private address in the home network may conflict with the address space assigned to the residential gateway WAN interface. Some of these issues are documented in draft-shirasaki-nat444-isp-shared-addr-02. Should a conflict occur, many residential gateways fail to forward IP traffic.