diff --git a/CHANGES b/CHANGES
index 8a50133630..00c2b49a38 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,6 @@
+4412.	[cleanup]	Make fixes for GCC 6. ISC_OFFSET_MAXIMUM macro was
+			removed. [RT #42721]
+
 4411.	[func]		"rndc dnstap -roll" automatically rolls the 
 			dnstap output file; the previous version is
 			saved with ".0" suffix, and earlier versions
diff --git a/bin/dig/dighost.c b/bin/dig/dighost.c
index 7a3126b2d5..d02f6c5ee7 100644
--- a/bin/dig/dighost.c
+++ b/bin/dig/dighost.c
@@ -2549,7 +2549,7 @@ setup_lookup(dig_lookup_t *lookup) {
 
 			/* Mask off last address byte */
 			if (addrl > 0 && (plen % 8) != 0)
-				addr[addrl - 1] &= ~0 << (8 - (plen % 8));
+				addr[addrl - 1] &= ~0U << (8 - (plen % 8));
 
 			/* family */
 			isc_buffer_putuint16(&b, family);
diff --git a/bin/named/client.c b/bin/named/client.c
index e0c3558d82..3faf6d2e11 100644
--- a/bin/named/client.c
+++ b/bin/named/client.c
@@ -2037,7 +2037,7 @@ process_ecs(ns_client_t *client, isc_buffer_t *buf, size_t optlen) {
 		optlen -= addrbytes;
 
 		if ((addrlen % 8) != 0) {
-			isc_uint8_t bits = ~0 << (8 - (addrlen % 8));
+			isc_uint8_t bits = ~0U << (8 - (addrlen % 8));
 			bits &= paddr[addrbytes - 1];
 			if (bits != paddr[addrbytes - 1])
 				return (DNS_R_OPTERR);
diff --git a/bin/named/logconf.c b/bin/named/logconf.c
index 3dad682397..5442e06399 100644
--- a/bin/named/logconf.c
+++ b/bin/named/logconf.c
@@ -135,6 +135,22 @@ channel_fromconf(const cfg_obj_t *channel, isc_logconfig_t *logconfig)
 				 cfg_tuple_get(fileobj, "versions");
 		isc_int32_t versions = ISC_LOG_ROLLNEVER;
 		isc_offset_t size = 0;
+		isc_uint64_t maxoffset;
+
+		/*
+		 * isc_offset_t is a signed integer type, so the maximum
+		 * value is all 1s except for the MSB.
+		 */
+		switch (sizeof(isc_offset_t)) {
+		case 4:
+			maxoffset = 0x7fffffffLLU;
+			break;
+		case 8:
+			maxoffset = 0x7fffffffffffffffLLU;
+			break;
+		default:
+			INSIST(0);
+		}
 
 		type = ISC_LOG_TOFILE;
 
@@ -145,7 +161,7 @@ channel_fromconf(const cfg_obj_t *channel, isc_logconfig_t *logconfig)
 			versions = ISC_LOG_ROLLINFINITE;
 		if (sizeobj != NULL &&
 		    cfg_obj_isuint64(sizeobj) &&
-		    cfg_obj_asuint64(sizeobj) < ISC_OFFSET_MAXIMUM)
+		    cfg_obj_asuint64(sizeobj) < maxoffset)
 			size = (isc_offset_t)cfg_obj_asuint64(sizeobj);
 		dest.file.stream = NULL;
 		dest.file.name = cfg_obj_asstring(pathobj);
diff --git a/bin/tools/mdig.c b/bin/tools/mdig.c
index 0c85ece689..9f63cd1523 100644
--- a/bin/tools/mdig.c
+++ b/bin/tools/mdig.c
@@ -606,7 +606,7 @@ sendquery(struct query *query, isc_task_t *task)
 				memmove(addr, &sin->sin_addr, 4);
 				if ((plen % 8) != 0)
 					addr[addrl-1] &=
-						~0 << (8 - (plen % 8));
+						~0U << (8 - (plen % 8));
 			} else {
 				family = 2;
 				sin6 = (struct sockaddr_in6 *) sa;
@@ -615,7 +615,7 @@ sendquery(struct query *query, isc_task_t *task)
 
 			/* Mask off last address byte */
 			if (addrl > 0 && (plen % 8) != 0)
-				addr[addrl - 1] &= ~0 << (8 - (plen % 8));
+				addr[addrl - 1] &= ~0U << (8 - (plen % 8));
 
 			/* family */
 			isc_buffer_putuint16(&b, family);
diff --git a/lib/dns/rdata/generic/opt_41.c b/lib/dns/rdata/generic/opt_41.c
index 353ac25e1b..cd02399472 100644
--- a/lib/dns/rdata/generic/opt_41.c
+++ b/lib/dns/rdata/generic/opt_41.c
@@ -158,7 +158,7 @@ fromwire_opt(ARGS_FROMWIRE) {
 				return (DNS_R_OPTERR);
 
 			if (addrbytes != 0U && (addrlen % 8) != 0) {
-				isc_uint8_t bits = ~0 << (8 - (addrlen % 8));
+				isc_uint8_t bits = ~0U << (8 - (addrlen % 8));
 				bits &= sregion.base[addrbytes - 1];
 				if (bits != sregion.base[addrbytes - 1])
 					return (DNS_R_OPTERR);
diff --git a/lib/isc/radix.c b/lib/isc/radix.c
index b85ce5a301..415a660dfd 100644
--- a/lib/isc/radix.c
+++ b/lib/isc/radix.c
@@ -122,8 +122,8 @@ _comp_with_mask(void *addr, void *dest, u_int mask) {
 		return (1);
 
 	if (memcmp(addr, dest, mask / 8) == 0) {
-		int n = mask / 8;
-		int m = ((~0) << (8 - (mask % 8)));
+		u_int n = mask / 8;
+		u_int m = ((~0U) << (8 - (mask % 8)));
 
 		if ((mask % 8) == 0 ||
 		    (((u_char *)addr)[n] & m) == (((u_char *)dest)[n] & m))
diff --git a/lib/isc/unix/include/isc/offset.h b/lib/isc/unix/include/isc/offset.h
index f28aa509da..d63ae99f9f 100644
--- a/lib/isc/unix/include/isc/offset.h
+++ b/lib/isc/unix/include/isc/offset.h
@@ -21,17 +21,4 @@
 
 typedef off_t isc_offset_t;
 
-/*%
- * POSIX says "Additionally, blkcnt_t and off_t are extended signed integral
- * types", so the maximum value is all 1s except for the high bit.
- * This definition is more complex than it really needs to be because it was
- * crafted to keep both the SunOS 5.6 and the HP/UX 11 compilers quiet about
- * integer overflow.  For example, though this is equivalent to just left
- * shifting 1 to the high bit and then inverting the bits, the SunOS compiler
- * is unhappy about shifting a positive "1" to negative in a signed integer.
- */
-#define ISC_OFFSET_MAXIMUM \
-	(~(((off_t)-1 >> (sizeof(off_t) * CHAR_BIT - 1)) \
-		      << (sizeof(off_t) * CHAR_BIT - 1)))
-
 #endif /* ISC_OFFSET_H */
diff --git a/lib/isc/win32/include/isc/offset.h b/lib/isc/win32/include/isc/offset.h
index ce25f57842..ef26ea96a9 100644
--- a/lib/isc/win32/include/isc/offset.h
+++ b/lib/isc/win32/include/isc/offset.h
@@ -19,17 +19,4 @@
 
 typedef _off_t isc_offset_t;
 
-/*
- * POSIX says "Additionally, blkcnt_t and off_t are extended signed integral
- * types", so the maximum value is all 1s except for the high bit.
- * This definition is more complex than it really needs to be because it was
- * crafted to keep both the SunOS 5.6 and the HP/UX 11 compilers quiet about
- * integer overflow.  For example, though this is equivalent to just left
- * shifting 1 to the high bit and then inverting the bits, the SunOS compiler
- * is unhappy about shifting a positive "1" to negative in a signed integer.
- */
-#define ISC_OFFSET_MAXIMUM \
-	(~(((off_t)-1 >> (sizeof(off_t) * CHAR_BIT - 1)) \
-		      << (sizeof(off_t) * CHAR_BIT - 1)))
-
 #endif /* ISC_OFFSET_H */