diff --git a/types/netmap/netmap.go b/types/netmap/netmap.go
index 558c74637..5bf1441ac 100644
--- a/types/netmap/netmap.go
+++ b/types/netmap/netmap.go
@@ -18,6 +18,10 @@
 	"tailscale.com/wgengine/filter"
 )
 
+// NetworkMap is the current state of the world.
+//
+// The fields should all be considered read-only. They might
+// alias parts of previous NetworkMap values.
 type NetworkMap struct {
 	// Core networking