VPI 
Field Although the whole UNI VPI range from 0 through 255 can be processed by the ADSL CPE, only VPI 0, 1, 2, 3, 4, and 5 are 
of importance for ATMF–25.6 user traffic, and VPI 8 is of importance for Ethernet user traffic. All ATM cells arriving with 
VPI values other than these values shall be silently discarded. Note there is one VP/VC which is not mentioned here
due to its special purpose, that is, loopback (described later). The values given above apply to both upstream and downstream 
directions, that is, no header translation is done.

VCI 
Field For VPIs 0 through 5 - ATMF-25.6 traffic, the ADSL CPE is transparent for the VCI fields in the ATM header, hence all 
VCI values from 0 up to 65536 can be used. In addition, the reserved VCI values 0 through 31 are also transparently relayed; 
the ADSL CPE acts as a true ATM VP cross–connect. For VPI 8 (Ethernet traffic), only VCI values 
35, 43,