diff --git a/detect-dns64-prefix.py b/detect-dns64-prefix.py
index e5bd179..1179ca4 100644
--- a/detect-dns64-prefix.py
+++ b/detect-dns64-prefix.py
@@ -2,6 +2,12 @@
 # Nico Schottelius, 2020-01-07
 # Detect the DNS64 prefix
 # Based on https://tools.ietf.org/html/draft-ietf-behave-nat64-discovery-heuristic-05
+#
+# How it works:
+# - ipv4only.arpa only has A records.
+# - a DNS64 server will add AAAA records
+# - we take this response (if any) and derive the IPv6 prefix from it
+#
 
 import dns.resolver
 import ipaddress