Document the VLAN-id 0 special case.

docs/add_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 both IPv4 and (where the ISP provides for it) IPv6.
 
+If `${vlan}` is 0, then no tagged VLAN interfaces will be created; however,
+the subnet address computations will still be performed as-if the vlan
+were created.  This means that `xy0` will use `0` or `00` for its subnet
+computations in IPv4 and IPv6 respectively.  If you don't intend to use
+VLANs for subnets, just use VLAN ID 0 when declaring networks.
+
 The IPv4 network will be a `/24` under `${v4_net_xy0}` if `${card}` is
 `xy0`, for example.  If `${v4_net_xy0}` is not set, then `${v4_net}` is
 used by default.  The 3rd octet will be `$(( ${vlan}/10 + ${vlan}%10 ))`.  For
@@ -50,6 +56,7 @@ IPv6, and VLAN ID.  Each can be visually compared to the others.)
 
 |VLAN|IPv4 (3rd octet)|IPv6 suffix (hex)|Example IPv4   |Example IPv6           |
 |---:|---------------:|----------------:|--------------:|----------------------:|
+|   0| 0              |  0              |192.168.0.0/24 |2001:db8:abcd:ef00::/64|
 |   7| 7              |  7              |192.168.7.0/24 |2001:db8:abcd:ef07::/64|
 | 101|11              | 11              |192.168.11.0/24|2001:db8:abcd:ef11::/64|
 | 205|25              | 25              |192.168.25.0/24|2001:db8:abcd:ef25::/64|