diff --git a/CHANGES b/CHANGES
index f5e7c40214..583b90a834 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,6 @@
+4524.	[bug]		The net zero test was broken causing IPv4 servers
+			with addresses ending in .0 to be rejected. [RT #43776]
+
 4523.	[doc]		Expand config doc for <querysource4> and
 			<querysource6>. [RT #43768]
 
diff --git a/lib/isc/netaddr.c b/lib/isc/netaddr.c
index a41baa2775..c60afbb297 100644
--- a/lib/isc/netaddr.c
+++ b/lib/isc/netaddr.c
@@ -418,15 +418,15 @@ isc_netaddr_issitelocal(isc_netaddr_t *na) {
 	}
 }
 
-#ifndef IN_ZERONET
-#define IN_ZERONET(x) (((x) & htonl(0xff000000U)) == 0)
-#endif
+#define ISC_IPADDR_ISNETZERO(i) \
+               (((isc_uint32_t)(i) & ISC__IPADDR(0xff000000)) \
+                == ISC__IPADDR(0x00000000))
 
 isc_boolean_t
 isc_netaddr_isnetzero(isc_netaddr_t *na) {
 	switch (na->family) {
 	case AF_INET:
-		return (ISC_TF(IN_ZERONET(na->type.in.s_addr)));
+		return (ISC_TF(ISC_IPADDR_ISNETZERO(na->type.in.s_addr)));
 	case AF_INET6:
 		return (ISC_FALSE);
 	default:
diff --git a/lib/isc/tests/Makefile.in b/lib/isc/tests/Makefile.in
index ab78ffc4e1..98f275c9a8 100644
--- a/lib/isc/tests/Makefile.in
+++ b/lib/isc/tests/Makefile.in
@@ -39,7 +39,8 @@ SRCS =		isctest.c taskpool_test.c socket_test.c hash_test.c \
 		lex_test.c radix_test.c \
 		sockaddr_test.c symtab_test.c task_test.c queue_test.c \
 		parse_test.c pool_test.c print_test.c regex_test.c \
-		safe_test.c time_test.c counter_test.c errno_test.c
+		safe_test.c time_test.c counter_test.c errno_test.c \
+		netaddr_test.c
 
 SUBDIRS =
 TARGETS =	taskpool_test@EXEEXT@ socket_test@EXEEXT@ hash_test@EXEEXT@ \
@@ -48,7 +49,7 @@ TARGETS =	taskpool_test@EXEEXT@ socket_test@EXEEXT@ hash_test@EXEEXT@ \
 		queue_test@EXEEXT@ parse_test@EXEEXT@ pool_test@EXEEXT@ \
 		print_test@EXEEXT@ regex_test@EXEEXT@ socket_test@EXEEXT@ \
 		safe_test@EXEEXT@ time_test@EXEEXT@ counter_test@EXEEXT@ \
-		errno_test@EXEEXT@
+		errno_test@EXEEXT@ netaddr_test@EXEEXT@
 
 @BIND9_MAKE_RULES@
 
@@ -121,6 +122,10 @@ errno_test@EXEEXT@: errno_test.@O@ ${ISCDEPLIBS}
 	${LIBTOOL_MODE_LINK} ${PURIFY} ${CC} ${CFLAGS} ${LDFLAGS} -o $@ \
 			errno_test.@O@ ${ISCLIBS} ${LIBS}
 
+netaddr_test@EXEEXT@: netaddr_test.@O@ ${ISCDEPLIBS}
+	${LIBTOOL_MODE_LINK} ${PURIFY} ${CC} ${CFLAGS} ${LDFLAGS} -o $@ \
+			netaddr_test.@O@ ${ISCLIBS} ${LIBS}
+
 unit::
 	sh ${top_srcdir}/unit/unittest.sh
 
diff --git a/lib/isc/tests/netaddr_test.c b/lib/isc/tests/netaddr_test.c
new file mode 100644
index 0000000000..767bb32473
--- /dev/null
+++ b/lib/isc/tests/netaddr_test.c
@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2016  Internet Systems Consortium, Inc. ("ISC")
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+/* ! \file */
+
+#include <config.h>
+
+#include <atf-c.h>
+
+#include <stdio.h>
+#include <string.h>
+
+#include <isc/netaddr.h>
+
+ATF_TC(isc_netaddr_isnetzero);
+ATF_TC_HEAD(isc_netaddr_isnetzero, tc) {
+	atf_tc_set_md_var(tc, "descr", "test isc_netaddr_isnetzero");
+}
+ATF_TC_BODY(isc_netaddr_isnetzero, tc) {
+	unsigned int i;
+	struct in_addr ina;
+	struct {
+		const char *address;
+		isc_boolean_t expect;
+	} tests[] = {
+		{ "0.0.0.0", ISC_TRUE },
+		{ "0.0.0.1", ISC_TRUE },
+		{ "0.0.1.2", ISC_TRUE },
+		{ "0.1.2.3", ISC_TRUE },
+		{ "10.0.0.0", ISC_FALSE },
+		{ "10.9.0.0", ISC_FALSE },
+		{ "10.9.8.0", ISC_FALSE },
+		{ "10.9.8.7", ISC_FALSE },
+		{ "127.0.0.0", ISC_FALSE },
+		{ "127.0.0.1", ISC_FALSE }
+	};
+	isc_boolean_t result;
+	isc_netaddr_t netaddr;
+
+	for (i = 0; i < sizeof(tests)/sizeof(tests[0]); i++) {
+		ina.s_addr = inet_addr(tests[i].address);
+		isc_netaddr_fromin(&netaddr, &ina);
+		result = isc_netaddr_isnetzero(&netaddr);
+		ATF_CHECK_EQ_MSG(result, tests[i].expect,
+				 "%s", tests[i].address);
+	}
+}
+
+/*
+ * Main
+ */
+ATF_TP_ADD_TCS(tp) {
+
+	ATF_TP_ADD_TC(tp, isc_netaddr_isnetzero);
+
+	return (atf_no_error());
+}