diff --git a/bin/tests/system/ifconfig.sh.in b/bin/tests/system/ifconfig.sh.in
index 76233d6184..298bd3f041 100755
--- a/bin/tests/system/ifconfig.sh.in
+++ b/bin/tests/system/ifconfig.sh.in
@@ -24,6 +24,10 @@
 # We also set the MTU on the 1500 bytes to match the default MTU on physical
 # interfaces, so we can properly test the cases with packets bigger than
 # interface MTU.
+#
+# See also org.isc.bind.system (a version of this script for use on macOS)
+# and testsock.pl (which checks the interfaces are configured)
+#
 
 top_srcdir=@abs_top_srcdir@
 
@@ -237,6 +241,10 @@ sequence() (
 # 'ns' in the namserver being configured.
 # 'int' interface identifier.
 #
+# See also `org.isc.bind.system`.
+#
+# This `max` setting is grepped out for use by testsock.pl
+#
 max=11
 case $1 in
     start|up|stop|down)
diff --git a/bin/tests/system/org.isc.bind.system b/bin/tests/system/org.isc.bind.system
index 276437ad6e..6c5a8cd4fe 100644
--- a/bin/tests/system/org.isc.bind.system
+++ b/bin/tests/system/org.isc.bind.system
@@ -11,12 +11,14 @@
 # See the COPYRIGHT file distributed with this work for additional
 # information regarding copyright ownership.
 
+# see also ifconfig.sh.in and testsock.pl
+
 ifup() {
 	/sbin/ifconfig lo0 10.53.$1.$3 alias
 	/sbin/ifconfig lo0 inet6 fd92:7065:b8e:${2}ff::${3} alias
 }
 
-for ns in 1 2 3 4 5 6 7 8 9 10
+for ns in 1 2 3 4 5 6 7 8 9 10 11
 do
 	ifup 0 ff $ns
 done
diff --git a/bin/tests/system/testsock.pl b/bin/tests/system/testsock.pl
index e9448ed380..5073a815fb 100755
--- a/bin/tests/system/testsock.pl
+++ b/bin/tests/system/testsock.pl
@@ -27,7 +27,15 @@ my @ids;
 if ($id != 0) {
 	@ids = ($id);
 } else {
-	@ids = (1..8);
+	my $fn = "ifconfig.sh.in";
+	open FH, "< $fn" or die "open < $fn: $!\n";
+	while (<FH>) {
+		@ids = (1..$1)
+		    if /^max=(\d+)\s*$/;
+	}
+	close FH;
+	die "could not find max IP address in $fn\n"
+	    unless @ids > 1;
 }
 
 foreach $id (@ids) {