Following information was contributed by Stephane Bortzmeyer <bortzmeyer at nic dot fr>
Be sure that IPv6 is loaded, either because it is compiled into the kernel or because the module is loaded. For the latest, three solutions, adding it to /etc/modules, using the pre-up trick shown later or using kmod (not detailed here).
Configure your interface. Here we assume eth0 and address (2001:0db8:1234:5::1:1). Edit /etc/network/interfaces:
iface eth0 inet6 static
pre-up modprobe ipv6
address 2001:0db8:1234:5::1:1
# To suppress completely autoconfiguration:
# up echo 0 > /proc/sys/net/ipv6/conf/all/autoconf
netmask 64
# The router is autoconfigured and has no fixed address.
# It is magically
# found. (/proc/sys/net/ipv6/conf/all/accept_ra). Otherwise:
#gateway 2001:0db8:1234:5::1
And you reboot or you just
# ifup --force eth0
and you have your static address.
Jean-Marc V. Liotier's HOWTO for Freenet6 & Debian Users (announced 24.12.2002 on mailinglist users@ipv6.org )