diff --git a/ggsn_tests/GGSN_Tests.ttcn b/ggsn_tests/GGSN_Tests.ttcn
index 2b7de96..1469062 100644
--- a/ggsn_tests/GGSN_Tests.ttcn
+++ b/ggsn_tests/GGSN_Tests.ttcn
@@ -20,16 +20,35 @@
 	const integer GTP1U_PORT := 2152;
 
 	modulepar {
+		/* Default IP addresses. May be overridden by GGSN_Tests configuration files. */
+
+		/* The SGSN simulated by TTCN3 will bind to these addresses for GTP control and GTP user planes. */
 		charstring m_bind_ip_gtpc := "127.23.42.1";
 		charstring m_bind_ip_gtpu := "127.23.42.1";
 
+		/* Addresses the GGSN which is being tested is listening on for SGSN connections. */
 		charstring m_ggsn_ip_gtpc := "127.0.0.6";
 		charstring m_ggsn_ip_gtpu := "127.0.0.6";
 
+		/*
+		 * Our tests expect to see these DNS servers in 'Create PDP context responses' sent by the GGSN.
+		 * These addresses must therefore match 'ip[v6] dns' options configured in osmo-ggsn.conf.
+		 *
+		 * These addresses are not expected to serve actual DNS requests. However, tests may expect to be
+		 * able to ping these addresses (currently, IPv4 addresses must respond with an ICMP 'echo reply',
+		 * and IPv6 addresses may respond with either ICMPv6 'echo reply' or 'destination unreachable').
+		 */
 		charstring m_ggsn_ip4_dns1 := "192.168.100.1"
 		charstring m_ggsn_ip4_dns2 := "8.8.8.8"
 		charstring m_ggsn_ip6_dns1 := "2001:4860:4860::8888"
 		charstring m_ggsn_ip6_dns2 := "2001:4860:4860::8844"
+
+		/*
+		 * Additional address ranges are defined in osmo-ggsn.conf from which addresses are assigned
+		 * to MS "behind" the simulated SGSN. These addresses appear on tun devices used by osmo-ggsn.
+		 * The tests expect to be able to send ping packets between any two simulated MS within the same
+		 * address range. This requires IP forwarding to be enabled on the corresponding tun interfaces.
+		 */
 	}
 
 	type set PdpContext {