diff --git a/conntrackt/models.py b/conntrackt/models.py
--- a/conntrackt/models.py
+++ b/conntrackt/models.py
@@ -210,7 +210,11 @@ class Communication(models.Model):
         """
 
         if self.source.entity == self.destination.entity:
-            raise ValidationError('Source and destination entities must differ.')
+            raise ValidationError('Source and destination entities are identical.')
+
+        if self.source.entity.project != self.destination.entity.project:
+            raise ValidationError('Source and destination entities do not belong to the same project')
+
         if (self.protocol.upper(), self.protocol.upper()) not in self.PROTOCOL_CHOICES:
             raise ValidationError('%s is not a supported protocol.' % self.protocol)