what is the enternal IP address of a cisco 1720 router?

Answer 1

You are referring to the loopback address, and you can set this to be whatever you want. You can't just hook up the router and point your browser to 192.168.1.1 for example like with a linksys. For example, to set the internal/loopback address, you can set the loopback0 address using these IOS commands from a console connection:
config term
int loop0
ip address 192.168.1.1 255.255.255.255
end
copy run start

A loopback interface is ALWAYS active and CANNOT fail, that is the reason they are used, they are always in the up/up state and this makes routing protocols such as OSPF and BGP much more stable, as opposed to using a real interface.

In addition, some routing protocols like OSPF will use the highest IP address configured on the router as the router ID, but you can override this behavior by configuring a loopback address.

Finally, you can use the loopback address as the "source interface" for traffic that is generated by the router, such as syslog packets, SNMP traps, and security related packets.

A typical config file's loopback related commands might look like this:
interface Loopback0
ip address 172.16.1.1 255.255.255.255
router ospf 100
router-id 172.16.1.1
ip tftp source-interface Loopback0
ip tacacs source-interface Loopback0
logging source-interface Loopback0
snmp-server trap-source Loopback0

Answer 2

It's whatever you set it to be; IOS doesn't provide a default. You might be able to find the address by plugging a cross-over cable directly into the interface and passively sniffing traffic using a sniffer like "ethereal". Or you could log into the router and use "wr t" to list the configuration.