Static IPv6 on Comcast network.

[thetibetantr]

I am having trouble stetting up a static IPb6 on Comcast.  I have a business account and they told me I was assigned 2603:XXX:YYY:ZZZ::/56.  Everything I have read online says that I should set the WAN interface to DHCPv6 and the prefix delegation size to "56".   When I do that I see the following on the Comcast modem:

Internet: Active
WAN IP Address (IPv6): 2001:558:6031:70:a59d:4c3:f6fc:ce79
WAN Default Gateway Address (IPv6): fe80::201:5cff:fea9:5246
Delegated prefix (IPv6): 2601:980:c300:4f0::/64
WAN Link Local Address (IPv6): fe80::fad2:acff:fe94:16a
DHCP Client (IPv6): Enabled


And the WAN interface shows:

# ifconfig em0
em0: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric 0 mtu 1500
description: WAN (wan)
options=4802028<VLAN_MTU,JUMBO_MTU,WOL_MAGIC,HWSTATS,MEXTPG>
ether 52:54:00:23:cf:af
inet 50.76.207.253 netmask 0xfffffff8 broadcast 50.76.207.255
inet6 fe80::5054:ff:fe23:cfaf%em0 prefixlen 64 scopeid 0x1
inet6 2601:980:c300:4f0:5054:ff:fe23:cfaf prefixlen 64 autoconf pltime 300 vltime 300
media: Ethernet autoselect (1000baseT <full-duplex>)
status: active
nd6 options=23<PERFORMNUD,ACCEPT_RTADV,AUTO_LINKLOCAL>


I have also set a VLAN interface's "IPV6 configuration Type" to "tracking". 

The VLAN only shows a link-local IPv6 address.

Is there a HOW-TO for configuring an static IPv6 that doesn't use DHCP to get the address?