Write page about NPTv6 (#134)

source/manual.rst

@@ -42,4 +42,5 @@ User Manual
    manual/opnsense_tools
    manual/diagnostics
    manual/monit
+   manual/nptv6
    manual/howtos

source/manual/nptv6.rst

@@ -0,0 +1,22 @@
+=====
+NPTv6
+=====
+
+Network Prefix Translation, shortened to NPTv6, is used to translate IPv6 addresses. A common usage for this
+is to translate global ("WAN") IPs to local ones. In this regard, it is similar to NAT, although NPTv6 can only be
+used to map addresses one-to-one, unlike NAT which typically translates one external IP to several internal ones.
+
+NPTv6 routes are listed at **Firewall->NAT->NPTv6**. New rules can be added by clicking **Add** in the upper right
+corner. A quick overview of the fields:
+
+============================= =======================================================================================================================================================================
+ Disabled                      Disables this rule without having to remove it.
+ Interface                     Which interface this rule should apply to. Most of the time, this will be the WAN interface.
+ Description                   A description to easily indentify the purpose of this rule in the overview.
+ **Internal IPv6 Prefix**
+ Source / Invert               Use this option to invert the sense of the source address match.
+ Source / Address              The internal (LAN) ULA IPv6 Prefix for the Network Prefix Translation. The prefix size specified for the internal IPv6 prefix will be applied to the external prefix.
+ **Destination IPv6 Prefix**
+ Destination / Invert          Use this option to invert the sense of the destination address match.
+ Destination / Address         The Global Unicast routable IPv6 prefix.
+============================= =======================================================================================================================================================================