diff --git a/sys/src/9/ip/ip.h b/sys/src/9/ip/ip.h
index e9f6fcf18..0635e1f0e 100644
--- a/sys/src/9/ip/ip.h
+++ b/sys/src/9/ip/ip.h
@@ -164,6 +164,12 @@ struct Ip4hdr
 	uchar	dst[4];		/* IP destination */
 };
 
+/*
+ *  cache for route and arp table lookups.
+ *  one per Conv so conversations with a
+ *  fixed target can avoid the per packet
+ *  lookup overhead.
+ */ 
 struct Routehint
 {
 	Route	*r;			/* last route used */