D-Link DSL-G256DG User Manual page 162

In this section you can perform loopback and linktrace test.
Target MAC: Enter the target MAC address
Linktrace TTL: Enter the link trace TTL.
Click the Send Loopback button to start the loopback test.
Click the Send Linktrace button to start the linktrace test.
Click the Save/Apply button to accept the changes made.
Web User Interface Configuration
D-Link DSL-G256DG VDSL Router User Manual
Page 162