diff --git a/TODO b/TODO
index 972afae7..2bf8eb3c 100644
--- a/TODO
+++ b/TODO
@@ -1,6 +1,5 @@
 Core
 ~~~~
-- config: when parsing prefix, check zero bits
 - config: try to unify expressions
 
 - static: check validity of route destination?
diff --git a/conf/confbase.Y b/conf/confbase.Y
index 7ac07fea..c6143012 100644
--- a/conf/confbase.Y
+++ b/conf/confbase.Y
@@ -40,6 +40,7 @@ CF_DECLS
   struct rt_show_data *ra;
   void *g;
   bird_clock_t time;
+  struct prefix px;
 }
 
 %token END CLI_MARKER INVALID_TOKEN
@@ -52,6 +53,7 @@ CF_DECLS
 
 %type <i> expr bool pxlen
 %type <time> datetime
+%type <px> prefix
 
 %nonassoc '=' '<' '>' '~' '.' GEQ LEQ NEQ
 %left '+' '-'
@@ -109,6 +111,13 @@ bool:
 
 /* Prefixes and netmasks */
 
+prefix:
+   IPA pxlen {
+     if (!ip_is_prefix($1, $2)) cf_error("Invalid prefix");
+     $$.addr = $1; $$.len = $2;
+   }
+ ;
+
 pxlen:
    '/' NUM {
      if ($2 < 0 || $2 > BITS_PER_IP_ADDRESS) cf_error("Invalid prefix length %d", $2);
diff --git a/filter/config.Y b/filter/config.Y
index 38ee16a6..343c1e07 100644
--- a/filter/config.Y
+++ b/filter/config.Y
@@ -42,7 +42,7 @@ CF_KEYWORDS(FUNCTION, PRINT, PRINTN, UNSET, RETURN,
 %type <f> filter filter_body where_filter
 %type <i> type break_command pair
 %type <e> set_item set_items switch_body
-%type <v> set_atom prefix prefix_s ipa
+%type <v> set_atom fprefix fprefix_s ipa
 %type <s> decls declsn one_decl function_params 
 %type <h> bgp_path
 %type <i> bgp_one
@@ -212,15 +212,18 @@ pair:
 /*
  * Complex types, their bison value is struct f_val
  */
-prefix_s:
-   IPA '/' NUM { $$.type = T_PREFIX; $$.val.px.ip = $1; $$.val.px.len = $3; if (ipa_nonzero(ipa_and($$.val.px.ip, ipa_not(ipa_mkmask($$.val.px.len))))) cf_error( "%I/%d is not really prefix\n", $$.val.px.ip, $$.val.px.len ); }
+fprefix_s:
+   IPA '/' NUM {
+     if (!ip_is_prefix($1, $3)) cf_error("Invalid network prefix: %I/%d", $1, $3);
+     $$.type = T_PREFIX; $$.val.px.ip = $1; $$.val.px.len = $3;
+   }
  ;
 
-prefix:
-   prefix_s { $$ = $1; }
- | prefix_s '+' { $$ = $1; $$.val.px.len |= LEN_PLUS; }
- | prefix_s '-' { $$ = $1; $$.val.px.len |= LEN_MINUS; }
- | prefix_s '{' NUM ',' NUM '}' { $$ = $1; $$.val.px.len |= LEN_RANGE | ($3 << 16) | ($5 << 8); }
+fprefix:
+   fprefix_s { $$ = $1; }
+ | fprefix_s '+' { $$ = $1; $$.val.px.len |= LEN_PLUS; }
+ | fprefix_s '-' { $$ = $1; $$.val.px.len |= LEN_MINUS; }
+ | fprefix_s '{' NUM ',' NUM '}' { $$ = $1; $$.val.px.len |= LEN_RANGE | ($3 << 16) | ($5 << 8); }
  ;
 
 ipa:
@@ -231,7 +234,7 @@ set_atom:
    NUM  { $$.type = T_INT; $$.val.i = $1; }
  | pair { $$.type = T_PAIR; $$.val.i = $1; }
  | ipa  { $$ = $1; }
- | prefix { $$ = $1; }
+ | fprefix { $$ = $1; }
  ; 
 
 set_item:
@@ -291,7 +294,7 @@ constant:
  | TEXT   { $$ = f_new_inst(); $$->code = 'c'; $$->aux = T_STRING; $$->a2.p = $1; }
  | pair   { $$ = f_new_inst(); $$->code = 'c'; $$->aux = T_PAIR;  $$->a2.i = $1; }
  | ipa	   { NEW_F_VAL; $$ = f_new_inst(); $$->code = 'C'; $$->a1.p = val; *val = $1; }
- | prefix_s {NEW_F_VAL; $$ = f_new_inst(); $$->code = 'C'; $$->a1.p = val; *val = $1; }
+ | fprefix_s {NEW_F_VAL; $$ = f_new_inst(); $$->code = 'C'; $$->a1.p = val; *val = $1; }
  | '[' set_items ']' { DBG( "We've got a set here..." ); $$ = f_new_inst(); $$->code = 'c'; $$->aux = T_SET; $$->a2.p = build_tree($2); DBG( "ook\n" ); }
  | ENUM	  { $$ = f_new_inst(); $$->code = 'c'; $$->aux = $1 >> 16; $$->a2.i = $1 & 0xffff; }
  | '/' bgp_path '/' { NEW_F_VAL; $$ = f_new_inst(); $$->code = 'C'; val->type = T_PATH_MASK; val->val.path_mask = $2; $$->a1.p = val; }
diff --git a/filter/filter.h b/filter/filter.h
index e86aa02b..a1bf5481 100644
--- a/filter/filter.h
+++ b/filter/filter.h
@@ -30,7 +30,7 @@ struct f_inst {		/* Instruction */
 #define arg1 a1.p
 #define arg2 a2.p
 
-struct prefix {
+struct f_prefix {
   ip_addr ip;
   int len;
 #define LEN_MASK 0xff
@@ -45,7 +45,7 @@ struct f_val {
   union {
     int i;
     /*    ip_addr ip; Folded into prefix */	
-    struct prefix px;
+    struct f_prefix px;
     char *s;
     struct f_tree *t;
     struct adata *ad;
diff --git a/lib/ip.h b/lib/ip.h
index 38f561b4..1cc74e81 100644
--- a/lib/ip.h
+++ b/lib/ip.h
@@ -41,9 +41,14 @@
 char *ip_scope_text(unsigned);
 
 /*
- *	Is it a valid network prefix?
+ *	Network prefixes
  */
 
+struct prefix {
+  ip_addr addr;
+  int len;
+};
+
 #define ip_is_prefix(a,l) (!ipa_nonzero(ipa_and(a, ipa_not(ipa_mkmask(l)))))
 
 /*
diff --git a/nest/config.Y b/nest/config.Y
index e5249ea4..f92c0541 100644
--- a/nest/config.Y
+++ b/nest/config.Y
@@ -124,8 +124,8 @@ debug_default:
 
 iface_patt:
    TEXT { this_ipatt->pattern = $1; this_ipatt->prefix = IPA_NONE; this_ipatt->pxlen = 0; }
- | IPA pxlen { this_ipatt->pattern = NULL; this_ipatt->prefix = $1; this_ipatt->pxlen = $2; }
- | TEXT IPA pxlen { this_ipatt->pattern = $1; this_ipatt->prefix = $2; this_ipatt->pxlen = $3; }
+ | prefix { this_ipatt->pattern = NULL; this_ipatt->prefix = $1.addr; this_ipatt->pxlen = $1.len; }
+ | TEXT prefix { this_ipatt->pattern = $1; this_ipatt->prefix = $2.addr; this_ipatt->pxlen = $2.len; }
  ;
 
 /* Direct device route protocol */
@@ -250,12 +250,11 @@ r_args:
      $$->filter = FILTER_ACCEPT;
      $$->table = config->master_rtc->table;
    }
- | r_args IPA pxlen {
+ | r_args prefix {
      $$ = $1;
      if ($$->pxlen != 256) cf_error("Only one prefix expected");
-     if (!ip_is_prefix($2, $3)) cf_error("Invalid prefix");
-     $$->prefix = $2;
-     $$->pxlen = $3;
+     $$->prefix = $2.addr;
+     $$->pxlen = $2.len;
    }
  | r_args TABLE SYM {
      $$ = $1;
diff --git a/proto/static/config.Y b/proto/static/config.Y
index 12d7934f..0197da36 100644
--- a/proto/static/config.Y
+++ b/proto/static/config.Y
@@ -34,12 +34,11 @@ static_proto:
  | static_proto stat_route ';'
  ;
 
-stat_route0: ROUTE IPA pxlen {
+stat_route0: ROUTE prefix {
      this_srt = cfg_allocz(sizeof(struct static_route));
      add_tail(&((struct static_config *) this_proto)->other_routes, &this_srt->n);
-     if (!ip_is_prefix($2, $3)) cf_error("Invalid network prefix: %I/%d", $2, $3);
-     this_srt->net = $2;
-     this_srt->masklen = $3;
+     this_srt->net = $2.addr;
+     this_srt->masklen = $2.len;
   }
  ;