diff --git a/RELNOTES b/RELNOTES
index 1b234329..55de1144 100644
--- a/RELNOTES
+++ b/RELNOTES
@@ -54,6 +54,9 @@ suggested fixes to <dhcp-users@isc.org>.
 
 			Changes since 4.0.0a1
 
+- Invalid CIDR representation for IPv6 subnets or ranges now checked
+  for when loading configuration.
+
 - Compilation on HP/UX has been repaired.  The changes should generally
   apply to any architecture that supplies SIOCGLIFCONF but does not
   use 'struct lifconf' structures to pass values.
diff --git a/common/inet.c b/common/inet.c
index 36dcf0f1..4f8f1182 100644
--- a/common/inet.c
+++ b/common/inet.c
@@ -35,7 +35,7 @@
 
 #ifndef lint
 static char copyright[] =
-"$Id: inet.c,v 1.13 2007/05/19 19:16:24 dhankins Exp $ Copyright (c) 2004,2005,2007 Internet Systems Consortium.  All rights reserved.\n";
+"$Id: inet.c,v 1.14 2007/06/19 17:06:03 shane Exp $ Copyright (c) 2004,2005,2007 Internet Systems Consortium.  All rights reserved.\n";
 #endif /* not lint */
 
 #include "dhcpd.h"
@@ -297,6 +297,66 @@ addr_and(struct iaddr *result, const struct iaddr *a1, const struct iaddr *a2) {
 	return !all_zero;
 }
 
+/*
+ * Check if a bitmask of the given length is valid for the address.
+ * This is not the case if any bits longer than the bitmask are 1.
+ *
+ * So, this is valid:
+ *
+ * 127.0.0.0/8
+ *
+ * But this is not:
+ *
+ * 127.0.0.1/8
+ *
+ * Because the final ".1" would get masked out by the /8.
+ */
+isc_boolean_t
+is_cidr_mask_valid(const struct iaddr *addr, int bits) {
+	int zero_bits;
+	int zero_bytes;
+	int i;
+	char byte;
+	int shift_bits;
+
+	/*
+	 * Check our bit boundaries.
+	 */
+	if (bits < 0) {
+		return ISC_FALSE;
+	}
+	if (bits > (addr->len * 8)) {
+		return ISC_FALSE;
+	}
+
+	/*
+	 * Figure out how many low-order bits need to be zero.
+	 */
+	zero_bits = (addr->len * 8) - bits;
+	zero_bytes = zero_bits / 8;
+
+	/* 
+	 * Check to make sure the low-order bytes are zero.
+	 */
+	for (i=1; i<=zero_bytes; i++) {
+		if (addr->iabuf[addr->len-i] != 0) {
+			return ISC_FALSE;
+		}
+	}
+
+	/* 
+	 * Look to see if any bits not in right-hand bytes are 
+	 * non-zero, by making a byte that has these bits set to zero 
+	 * comparing to the original byte. If these two values are 
+	 * equal, then the right-hand bits are zero, and we are 
+	 * happy.
+	 */
+	shift_bits = zero_bits % 8;
+	if (shift_bits == 0) return ISC_TRUE;
+	byte = addr->iabuf[addr->len-zero_bytes-1];
+	return (((byte >> shift_bits) << shift_bits) == byte);
+}
+
 /*
  * range2cidr
  *
diff --git a/includes/dhcpd.h b/includes/dhcpd.h
index 87209547..9735bdf3 100644
--- a/includes/dhcpd.h
+++ b/includes/dhcpd.h
@@ -2321,6 +2321,7 @@ int addr_or(struct iaddr *result,
 	    const struct iaddr *a1, const struct iaddr *a2);
 int addr_and(struct iaddr *result, 
 	     const struct iaddr *a1, const struct iaddr *a2);
+isc_boolean_t is_cidr_mask_valid(const struct iaddr *addr, int bits);
 isc_result_t range2cidr(struct iaddrcidrnetlist **result,
 			const struct iaddr *lo, const struct iaddr *hi);
 isc_result_t free_iaddrcidrnetlist(struct iaddrcidrnetlist **result);
diff --git a/server/confpars.c b/server/confpars.c
index c4fb0134..38361920 100644
--- a/server/confpars.c
+++ b/server/confpars.c
@@ -34,7 +34,7 @@
 
 #ifndef lint
 static char copyright[] =
-"$Id: confpars.c,v 1.167 2007/06/08 14:58:20 dhankins Exp $ Copyright (c) 2004-2007 Internet Systems Consortium.  All rights reserved.\n";
+"$Id: confpars.c,v 1.168 2007/06/19 17:06:03 shane Exp $ Copyright (c) 2004-2007 Internet Systems Consortium.  All rights reserved.\n";
 #endif /* not lint */
 
 #include "dhcpd.h"
@@ -2640,6 +2640,12 @@ parse_subnet6_declaration(struct parse *cfile, struct shared_network *share) {
 		return;
 	}
 
+	if (!is_cidr_mask_valid(&subnet->net, subnet->prefix_len)) {
+		parse_warn(cfile, "New subnet mask too short.");
+		skip_to_semi(cfile);
+		return;
+	}
+
 	/* 
 	 * Create a netmask. 
 	 */
@@ -3703,6 +3709,11 @@ parse_address_range6(struct parse *cfile, struct group *group) {
 			skip_to_semi(cfile);
 			return;
 		}
+		if (!is_cidr_mask_valid(&lo, bits)) {
+			parse_warn(cfile, "network mask too short");
+			skip_to_semi(cfile);
+			return;
+		}
 
 		add_ipv6_pool_to_shared_network(share, &lo, bits);