Draytek Vigor2830 Series User Manual page 109

IPv4 Mask Length
6rd Prefix
6rd Prefix Length
After finished the above settings, click OK to save the settings.
Below shows an example for successful IPv6 connection based on 6rd mode.
Type a number of high-order bits that are identical across
all CE IPv4 addresses within a given 6rd domain.
It may be any value between 0 and 32.
Type the 6rd IPv6 address.
Type the IPv6 prefix length for the 6rd IPv6 prefix in
number of bits.
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Vigor2830 Series User's Guide