A lot more documentation.

docs/add_net.md

@@ -41,6 +41,42 @@ skip provisioning the specified network with any ISP's DHCPv6 `IA_PD` request.
 This calls [`add_v4_net`](add_v4_net.md) and [`add_v6_net`](add_v6_net.md) and
 passes all of its arguments.
 
+## Subnet Calculation
+
+(It is strongly recommended that the 10s digit and 1000s digit of the VLAN ID be
+kept as `0`, as those digits are reserved for future improvements.  While this
+limits the system to 100 vlans, it provides 1:1 visual alignment between IPv4,
+IPv6, and VLAN ID.  Each can be visually compared to the others.)
+
+|VLAN|IPv4 (3rd octet)|IPv6 suffix (hex)|Example IPv4   |Example IPv6            |
+|---:|---------------:|----------------:|--------------:|-----------------------:|
+|   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|
+| 900|90              | 90              |192.168.90.0/24|2001:db8:abcd:ef25:::/64|
+
+## Options
+
+ - `no_ula` - Do not create a Unique Local Address for this network (on the IPv6
+   side).
+ - `isp=${NAME}` - Attach this network to a specified ISP's configuration
+ - `no_isp` - This network should not be associated with any ISP (declines to
+   add network to DHCPv6 PD, among other things).
+
+## Examples
+
+#### Basic home LAN
+`add_net abc2 100 main`
+
+#### Guest network with no ULA
+`add_net xy2 102 guest no_ula`
+
+#### IoT network with no ULA and no ISP prefix delegation
+`add_net xy2 103 iot no_ula no_isp`
+
+#### Management network on a different ISP PD
+`add_net xy2 402 mgmt isp=fios_1`
+
 ## See Also
 
  - [`add_v4_net`](add_v4_net.md)