diff --git a/network-query.rkt b/network-query.rkt
index b8b4bb3..ed47158 100644
--- a/network-query.rkt
+++ b/network-query.rkt
@@ -71,12 +71,12 @@
 (define first-timeout 3) ;; seconds
 
 ;; A NetworkRequest is a (network-request UdpAddress Question
-;; DomainName NEListOf<UdpAddress> UniqueID) representing the
+;; DomainName NEListOf<DomainName> UniqueID) representing the
 ;; parameters used to start and process a network query.
 (struct network-request (client-socket
 			 question
 			 zone-origin
-			 server-ips
+			 server-names
 			 unique-id)
 	#:transparent)
 
@@ -85,19 +85,15 @@
 (struct network-reply (unique-id answer) #:transparent)
 
 ;; A NetworkQueryState is a (network-query-state NetworkRequest
-;; Integer ListOf<UdpAddress>), representing an in-progress DNS
-;; network query.
+;; Integer Map<DomainName,ListOf<UdpAddress>> ListOf<UdpAddress>
+;; ListOf<DomainName>), representing an in-progress DNS network query.
 (struct network-query-state (request
 			     timeout
-			     remaining-addresses)
+			     known-addresses
+			     remaining-addresses
+			     remaining-names)
 	#:transparent)
 
-;; A ResolvingNameservers is a (resolving-nameservers NetworkRequest
-;; Integer), representing an in-progress nameserver resolution
-;; operation that must be completed before the actual query can
-;; continue.
-(struct resolving-nameservers (request remaining-count) #:transparent)
-
 ;; seconds -> Maybe<seconds>
 (define (next-timeout timeout)
   (case timeout
@@ -149,50 +145,60 @@
 
 ;; network-query : UdpAddress Question DomainName NEListOf<DomainName> UniqueId -> OsProcess
 (define (network-query s q zone-origin server-names unique-id)
-  (os-big-bang (resolving-nameservers (network-request s q zone-origin '() unique-id)
-				      (length server-names))
-    (map (lambda (nameserver-name)
-	   (define subscription-id (list 'nameserver-name-resolution nameserver-name))
-	   (define subq (question nameserver-name 'a 'in)) ;; TODO: 'aaaa ?
-	   (list (send-message subq)
-		 (subscribe subscription-id
-		   (message-handlers (resolving-nameservers req remaining)
-		     [(answered-question (== subq) ans)
-		      (define new-ips
-			(append (network-request-server-ips req)
-				(map make-dns-address
-				     (set->list (extract-addresses nameserver-name ans)))))
-		      (define new-req (struct-copy network-request req [server-ips new-ips]))
-		      (define new-remaining (- remaining 1))
-		      (extend-transition
-			  (if (zero? new-remaining)
-			      (try-next-server (network-query-state new-req
-								    first-timeout
-								    new-ips))
-			      (transition (resolving-nameservers new-req new-remaining)))
-			(unsubscribe subscription-id))]))))
-	 server-names)))
+  (os-big-bang/transition
+   (try-next-server (network-query-state (network-request s q zone-origin server-names unique-id)
+					 first-timeout
+					 (hash)
+					 '()
+					 server-names))))
 
 (define (try-next-server w)
   (match w
-    [(network-query-state req timeout '())
+    [(network-query-state req #f _ _ _)
+     ;; No more timeouts to try, so give up.
+     (on-answer w (empty-complete-answer))]
+    [(network-query-state req timeout _ '() '())
      ;; No more addresses to try with this timeout. Refill the list
      ;; and bump the timeout and retry.
      ;; TODO: randomize ordering of servers in list. (Don't forget the initial fill.)
      (try-next-server (struct-copy network-query-state w
 				   [timeout (next-timeout timeout)]
-				   [remaining-addresses (network-request-server-ips req)]))]
-    [(network-query-state req #f _)
-     ;; No more timeouts to try, so give up.
-     (on-answer w (empty-complete-answer))]
-    [(network-query-state req timeout (cons current-address remaining-addresses))
+				   [remaining-addresses '()]
+				   [remaining-names (network-request-server-names req)]))]
+    [(network-query-state _ _ known-addresses '() (cons current-name remaining-names))
+     (if (hash-has-key? known-addresses current-name)
+	 (try-next-server (struct-copy network-query-state w
+				       [remaining-addresses (hash-ref known-addresses
+								      current-name)]
+				       [remaining-names remaining-names]))
+	 (let ((subscription-id (list 'nameserver-name-resolution current-name))
+	       (subq (question current-name 'a 'in))) ;; TODO: 'aaaa ?
+	   (transition (struct-copy network-query-state w [remaining-names remaining-names])
+	     (send-message subq)
+	     (subscribe subscription-id
+	       (message-handlers w
+		 [(answered-question (== subq) ans)
+		  (define ips
+		    (map make-dns-address (set->list (extract-addresses current-name ans))))
+		  (extend-transition
+		      (try-next-server (struct-copy network-query-state w
+						    [known-addresses (hash-set known-addresses
+									       current-name
+									       ips)]
+						    [remaining-addresses ips]))
+		    (unsubscribe subscription-id))])))))]
+    [(network-query-state req timeout _ (cons current-ip remaining-ips) _)
      (define rpc-id (gensym 'network-query/allocate-query-id))
      (transition w
        (send-message `(request ,rpc-id allocate-query-id))
        (subscribe rpc-id
 	 (message-handlers w
 	   [`(reply ,(== rpc-id) ,id)
-	    (extend-transition (send-request req id timeout current-address remaining-addresses)
+	    (extend-transition (send-request (struct-copy network-query-state w
+							  [remaining-addresses remaining-ips])
+					     id
+					     timeout
+					     current-ip)
 	      (unsubscribe rpc-id))])))]))
 
 (define (on-answer w ans)
@@ -200,12 +206,12 @@
     (send-message (network-reply (network-request-unique-id (network-query-state-request w))
 				 ans))))
 
-(define (send-request req query-id timeout server-ip remaining-ips)
-  (match-define (network-request s q zone-origin _ _) req)
+(define (send-request w query-id timeout server-ip)
+  (match-define (network-request s q zone-origin _ _) (network-query-state-request w))
   (define query (make-dns-query-message q query-id))
   (define subscription-id (list s query-id))
   (define start-time (current-inexact-milliseconds))
-  (transition (network-query-state req timeout remaining-ips)
+  (transition w
     (send-message (dns-request query s server-ip))
     (send-message (set-timer subscription-id (* timeout 1000) #t))
     (subscribe subscription-id