In a typical enterprise network, the NAT router has inside and outside interfaces, the inside interface is connected to the LAN and the outside interface is connected to the ISP. Is the NAT router connected directly to the ISP router or it is connected to a T1 ISP circuit before it reaches the ISP backbone/core network?
Routers are layer-3 devices. They communicate with other layer-3 devices (routers, NAT devices, layer-3 firewalls etc.) over layer-2 infrastructure (T1, ethernet etc.).
ISP networks can be quite complex. Here in The Netherlands we have many DSL connections like this:
- Customer edge router (can be a NAT device) connects using PPPoA (over ATM/ADSL) to a DSLAM
- The DSLAM converts PPPoA (over ATM/DSL) to PPPoE (over Ethernet)
- The traffic probably travels over several ethernet switches to the ISP's edge
- A BRAS (Broadband Remote Access Server) at the ISP terminates the PPP connection and routes the traffic to the ISP's core network
- The core network brings the traffic to the transit and peering routers of the ISP
- These routers speak BGP to arrange routing with other ISPs and networks
But for enterprise networks it is also common to connect directly over ethernet to the ISP. Maybe speaking BGP between the ISP and the enterprise.