diff --git a/networks.rkt b/networks.rkt
index 5f59387..951a4b8 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -18,7 +18,9 @@
  (contract-out [struct tbf/state ([weights (hash/c variable? number?)]
                                   [threshold number?])]
                [struct dynamics ([network network?]
-                                 [mode mode?])])
+                                 [mode mode?])]
+               [struct network ([functions (hash/c variable? procedure?)]
+                                [domains domain-mapping/c])])
  ;; Functions
  (contract-out [update (-> network? state? (set/c variable? #:kind 'dont-care) state?)]
                [make-state (-> (listof (cons/c symbol? any/c)) state?)]
@@ -173,13 +175,10 @@
 ;;; state.
 (define update-function/c (-> state? any/c))
 
-;;; A network is a mapping from its variables to its update functions.
-;;;
-;;; Note that the domains of the variables of the network are not part
-;;; of the network definition.  This is because the variables of some
-;;; networks may have infinite domains, which can be restricted in
-;;; multiple different ways.
-(define network? (hash/c variable? procedure?))
+;;; A network consists of a mapping from its variables to its update
+;;; variables, as a well as of a mapping from its variables to
+;;; their domains.
+(struct network (functions domains))
 
 ;;; Given a state s updates all the variables from xs.
 (define (update network s xs)