The following should probably be obvious, but I had a surprisingly hard time figuring out the official Cisco documentation. The scenario is as follows: a gateway Cisco router provides internet access via NAT for, say, your office. It therefore has an external interface (called Dialer0, for example) which is connected to the uplink (this could e.g. be a DSL line), and an internal ethernet interface connected to the internal office network: ^ | | Dialer0: 87.
