diff --git a/src/core/net/ip6.cpp b/src/core/net/ip6.cpp
index ba3b73c4b..b617dc099 100644
--- a/src/core/net/ip6.cpp
+++ b/src/core/net/ip6.cpp
@@ -1123,28 +1123,15 @@ const NetifUnicastAddress *Ip6::SelectSourceAddress(MessageInfo &aMessageInfo)
                     rvalPrefixMatched = candidatePrefixMatched;
                 }
             }
-            else if (rvalAddr->GetScope() == Address::kRealmLocalScope)
+            else if ((rvalAddr->GetScope() == Address::kRealmLocalScope) &&
+                     (addr->GetScope() == Address::kRealmLocalScope))
             {
-                // Additional rule: Prefer appropriate realm local address
-                if (overrideScope > Address::kRealmLocalScope)
+                // Additional rule: Prefer EID
+                if (rvalAddr->GetAddress().IsRoutingLocator())
                 {
-                    if (rvalAddr->GetAddress().IsRoutingLocator())
-                    {
-                        // Prefer EID if destination is not realm local.
-                        rvalAddr          = addr;
-                        rvalIface         = candidateId;
-                        rvalPrefixMatched = candidatePrefixMatched;
-                    }
-                }
-                else
-                {
-                    if (candidateAddr->IsRoutingLocator())
-                    {
-                        // Prefer RLOC if destination is realm local.
-                        rvalAddr          = addr;
-                        rvalIface         = candidateId;
-                        rvalPrefixMatched = candidatePrefixMatched;
-                    }
+                    rvalAddr          = addr;
+                    rvalIface         = candidateId;
+                    rvalPrefixMatched = candidatePrefixMatched;
                 }
             }
             else if (addr->mPreferred && !rvalAddr->mPreferred)
diff --git a/src/core/thread/network_diagnostic.cpp b/src/core/thread/network_diagnostic.cpp
index d753ad9a2..25580c59a 100644
--- a/src/core/thread/network_diagnostic.cpp
+++ b/src/core/thread/network_diagnostic.cpp
@@ -124,6 +124,7 @@ otError NetworkDiagnostic::SendDiagnosticGet(const Ip6::Address &aDestination,
         SuccessOrExit(error = message->Append(aTlvTypes, aCount));
     }
 
+    messageInfo.SetSockAddr(GetNetif().GetMle().GetMeshLocal16());
     messageInfo.SetPeerAddr(aDestination);
     messageInfo.SetPeerPort(kCoapUdpPort);
     messageInfo.SetInterfaceId(netif.GetInterfaceId());
@@ -517,6 +518,7 @@ void NetworkDiagnostic::HandleDiagnosticGetQuery(Coap::Header &          aHeader
 
     VerifyOrExit((message = netif.GetCoap().NewMessage(header)) != NULL, error = OT_ERROR_NO_BUFS);
 
+    messageInfo.SetSockAddr(GetNetif().GetMle().GetMeshLocal16());
     messageInfo.SetPeerAddr(aMessageInfo.GetPeerAddr());
     messageInfo.SetPeerPort(kCoapUdpPort);
     messageInfo.SetInterfaceId(netif.GetInterfaceId());
@@ -631,6 +633,7 @@ otError NetworkDiagnostic::SendDiagnosticReset(const Ip6::Address &aDestination,
         SuccessOrExit(error = message->Append(aTlvTypes, aCount));
     }
 
+    messageInfo.SetSockAddr(GetNetif().GetMle().GetMeshLocal16());
     messageInfo.SetPeerAddr(aDestination);
     messageInfo.SetPeerPort(kCoapUdpPort);
     messageInfo.SetInterfaceId(netif.GetInterfaceId());