Loopback-Addr.

Sender address sent with the ping; this is also the destination for the answering ping.

Note: If the list of IP networks or loopback addresses contains an entry named 'DMZ' then the associated IP address will be used.
SNMP ID:
2.2.23.8 
Telnet path:
Setup > WAN > Polling-Table
Possible values:

Name of the IP network whose address should be used, or any valid IP address 

Special values:
INT
for the address of the first intranet
DMZ
for the address of the first DMZ
LB0 to LBF
for the 16 loopback addresses