Document the VLAN-id 0 special case.

docs/add_v6_net.md

@@ -16,6 +16,12 @@ Adds an internal network segment with the specified vlan on the specified
 card.  The created vlan-adaptor is put into the specified group.  It is setup
 for only IPv6.
 
+If `${vlan}` is 0, then no tagged VLAN interfaces will be created; however,
+the subnet address computations will still be performed as-if the vlan
+were created.  This means that `xy0` will use `00` for its subnet
+computations in IPv6.  If you don't intend to use
+VLANs for subnets, just use VLAN ID 0 when declaring networks.
+
 The IPv6 network will make `$(( ${vlan}/10 + ${vlan}%10 ))` into hex digits
 to be appended to the ISP's IPv6 prefix.  This makes it compatible with the
 most common case of `/56` for most ISPs.  If `no_ula` is specified in the
@@ -46,6 +52,7 @@ IPv6 and VLAN ID.  Each can be visually compared to the other.)
 
 |VLAN|IPv6 suffix (hex)|Example IPv6           |
 |---:|----------------:|----------------------:|
+|   0|  0              |2001:db8:abcd:ef00::/64|
 |   7|  7              |2001:db8:abcd:ef07::/64|
 | 101| 11              |2001:db8:abcd:ef11::/64|
 | 205| 25              |2001:db8:abcd:ef25::/64|