Core: convert IP addresses to reverse zone (#838)

* Core: convert IP addresses to reverse zone

If we see IP/mask syntax and the mask mod 8 == 0 we assume a reverse
zone and convert to in-addr or .arpa.

* typos

* integration test

* Addr is not used

* core: clean up normalize

Create a SplitHostPort function that can be used both from normalize.go
and address.go. This removes some (not all!) duplication between the
both and makes it work with reverse address notations.

* More tests

1 files changed, 13 insertions, 0 deletions

diff --git a/README.md b/README.md
index 4d9af837f..2533d0721 100644
--- a/README.md
+++ b/README.md
@@ -168,6 +168,19 @@ example.org {
 }
 ~~~
 
+IP addresses are also allowed. They are automatically converted to reverse zones:
+
+~~~ txt
+10.0.0.0/24 {
+    # ...
+}
+~~~
+Means you are authoritative for `0.0.10.in-addr.arpa.`. 
+
+The netmask must be dividable by 8, if it is not the reverse conversion is not done. This also works
+for IPv6 addresses. If for some reason you want to serve a zone named `10.0.0.0/24` add the closing
+dot: `10.0.0.0/24.` as this also stops the conversion.
+
 Listening on TLS and for gRPC? Use:
 
 ~~~ txt