diff --git a/chains/README.md b/chains/README.md
index dbb910de..28c994e2 100644
--- a/chains/README.md
+++ b/chains/README.md
@@ -1,5 +1,9 @@
 # Directory Layout
 
+Note: the host name used in the user's browser (displayed in the address bar) to load the explorer app determines which of the chain configuration directories is used.
+If the host name contains the substring "testnet" (for example: "https://mytestnetwork.example.com") then chain configuration files in the `testnet` directory (only) are loaded. Conversely if the host name does not contain that substring then chain configuration files in the `mainnet` directory (only) are loaded.
+Remember to bear this behavior in mind when selecting a DNS host name for self hosting the explorer.
+
 ** if you want to list your blockchain on ping.pub, please submit your configuration on https://github.com/ping-pub/ping.pub.git **
 
 - Submit configs for mainnet, go to https://github.com/ping-pub/explorer/tree/master/chains/mainnet