diff --git a/BUILDING.md b/BUILDING.md
index 63ddf8e7f4ff2a..5f81df8402c091 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -159,7 +159,7 @@ explains how to install all prerequisites.
 * GNU Make 3.81 or newer
 
 On macOS, install the `Xcode Command Line Tools` by running
-`xcode-select --install`. Alternatively, if you already have the full Xcode
+`xcode-select --install`. Alternatively, if you already have the full XcodeI
 installed, you can find them under the menu `Xcode -> Open Developer Tool ->
 More Developer Tools...`. This step will install `clang`, `clang++`, and
 `make`.
@@ -253,6 +253,16 @@ $ ./node ./test/parallel/test-stream2-transform.js
 Remember to recompile with `make -j4` in between test runs if you change code in
 the `lib` or `src` directories.
 
+The tests attempt to detect support for IPv6 and exclude IPv6 tests if
+appropriate. If your main interface has IPv6 addresses, then your
+loopback interface must also have '::1' enabled. For some default installations
+on Ubuntu that does not seem to be the case. To enable '::1' on the
+loopback interface on Ubuntu:
+
+```bash
+sudo sysctl -w net.ipv6.conf.lo.disable_ipv6=0
+```
+
 #### Running Coverage
 
 It's good practice to ensure any code you add or change is covered by tests.