diff --git a/src/Expr.cc b/src/Expr.cc
index d2d0cbf118..6f037e63c5 100644
--- a/src/Expr.cc
+++ b/src/Expr.cc
@@ -48,6 +48,7 @@ const char* expr_name(ExprTag t)
 		"-=",
 		"*",
 		"/",
+		"/", // mask operator
 		"%",
 		"&",
 		"|",
@@ -1918,14 +1919,27 @@ DivideExpr::DivideExpr(ExprPtr arg_op1, ExprPtr arg_op2)
 	else if ( BothArithmetic(bt1, bt2) )
 		PromoteType(max_type(bt1, bt2), is_vector(op1) || is_vector(op2));
 
-	else if ( bt1 == TYPE_ADDR && ! is_vector(op2) && (bt2 == TYPE_COUNT || bt2 == TYPE_INT) )
-		SetType(base_type(TYPE_SUBNET));
-
 	else
 		ExprError("requires arithmetic operands");
 	}
 
-ValPtr DivideExpr::AddrFold(Val* v1, Val* v2) const
+MaskExpr::MaskExpr(ExprPtr arg_op1, ExprPtr arg_op2)
+	: BinaryExpr(EXPR_MASK, std::move(arg_op1), std::move(arg_op2))
+	{
+	if ( IsError() )
+		return;
+
+	TypeTag bt1, bt2;
+	if ( ! get_types_from_scalars_or_vectors(this, bt1, bt2) )
+		return;
+
+	if ( bt1 == TYPE_ADDR && ! is_vector(op2) && (bt2 == TYPE_COUNT || bt2 == TYPE_INT) )
+		SetType(base_type(TYPE_SUBNET));
+	else
+		ExprError("requires address LHS and count/int RHS");
+	}
+
+ValPtr MaskExpr::AddrFold(Val* v1, Val* v2) const
 	{
 	uint32_t mask;
 
diff --git a/src/Expr.h b/src/Expr.h
index 712f386f17..0cfdf84a2c 100644
--- a/src/Expr.h
+++ b/src/Expr.h
@@ -52,6 +52,7 @@ enum ExprTag : int
 	EXPR_REMOVE_FROM,
 	EXPR_TIMES,
 	EXPR_DIVIDE,
+	EXPR_MASK,
 	EXPR_MOD,
 	EXPR_AND,
 	EXPR_OR,
@@ -819,6 +820,15 @@ public:
 	ExprPtr Duplicate() override;
 	bool WillTransform(Reducer* c) const override;
 	ExprPtr Reduce(Reducer* c, StmtPtr& red_stmt) override;
+	};
+
+class MaskExpr final : public BinaryExpr
+	{
+public:
+	MaskExpr(ExprPtr op1, ExprPtr op2);
+
+	// Optimization-related:
+	ExprPtr Duplicate() override;
 
 protected:
 	ValPtr AddrFold(Val* v1, Val* v2) const override;
diff --git a/src/parse.y b/src/parse.y
index b1df9ce7a2..c469ec93e4 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -633,7 +633,10 @@ expr:
 	|	expr '/' expr
 			{
 			set_location(@1, @3);
-			$$ = new DivideExpr({AdoptRef{}, $1}, {AdoptRef{}, $3});
+			if ( $1->GetType()->Tag() == TYPE_ADDR )
+				$$ = new MaskExpr({AdoptRef{}, $1}, {AdoptRef{}, $3});
+			else
+				$$ = new DivideExpr({AdoptRef{}, $1}, {AdoptRef{}, $3});
 			}
 
 	|	expr '%' expr
diff --git a/src/script_opt/CPP/Exprs.cc b/src/script_opt/CPP/Exprs.cc
index c8e60b6f28..8079c49a7b 100644
--- a/src/script_opt/CPP/Exprs.cc
+++ b/src/script_opt/CPP/Exprs.cc
@@ -80,6 +80,7 @@ string CPPCompile::GenExpr(const Expr* e, GenType gt, bool top_level)
 		case EXPR_TIMES:
 			return GenBinary(e, gt, "*", "mul");
 		case EXPR_DIVIDE:
+		case EXPR_MASK: // later code will split into addr masking
 			return GenBinary(e, gt, "/", "div");
 		case EXPR_MOD:
 			return GenBinary(e, gt, "%", "mod");
@@ -1060,7 +1061,7 @@ string CPPCompile::GenBinaryAddr(const Expr* e, GenType gt, const char* op)
 	{
 	auto v1 = GenExpr(e->GetOp1(), GEN_DONT_CARE) + "->AsAddr()";
 
-	if ( e->Tag() == EXPR_DIVIDE )
+	if ( e->Tag() == EXPR_MASK )
 		{
 		auto gen = string("addr_mask__CPP(") + v1 + ", " + GenExpr(e->GetOp2(), GEN_NATIVE) + ")";
 
diff --git a/src/script_opt/Expr.cc b/src/script_opt/Expr.cc
index 3a4be6cb13..83480680e7 100644
--- a/src/script_opt/Expr.cc
+++ b/src/script_opt/Expr.cc
@@ -253,6 +253,7 @@ bool Expr::IsFieldAssignable(const Expr* e) const
 		case EXPR_SUB:
 		case EXPR_TIMES:
 		case EXPR_DIVIDE:
+		case EXPR_MASK:
 		case EXPR_MOD:
 		case EXPR_AND:
 		case EXPR_OR:
@@ -1091,20 +1092,24 @@ ExprPtr DivideExpr::Duplicate()
 
 bool DivideExpr::WillTransform(Reducer* c) const
 	{
-	return GetType()->Tag() != TYPE_SUBNET && op2->IsOne();
+	return op2->IsOne();
 	}
 
 ExprPtr DivideExpr::Reduce(Reducer* c, StmtPtr& red_stmt)
 	{
-	if ( GetType()->Tag() != TYPE_SUBNET )
-		{
-		if ( op2->IsOne() )
-			return op1->ReduceToSingleton(c, red_stmt);
-		}
+	if ( op2->IsOne() )
+		return op1->ReduceToSingleton(c, red_stmt);
 
 	return BinaryExpr::Reduce(c, red_stmt);
 	}
 
+ExprPtr MaskExpr::Duplicate()
+	{
+	auto op1_d = op1->Duplicate();
+	auto op2_d = op2->Duplicate();
+	return SetSucc(new MaskExpr(op1_d, op2_d));
+	}
+
 ExprPtr ModExpr::Duplicate()
 	{
 	auto op1_d = op1->Duplicate();
diff --git a/src/script_opt/ZAM/Ops.in b/src/script_opt/ZAM/Ops.in
index 09df3828b4..0a774fa4f4 100644
--- a/src/script_opt/ZAM/Ops.in
+++ b/src/script_opt/ZAM/Ops.in
@@ -490,7 +490,16 @@ eval-pre	if ( $2 == 0 )
 			break;
 			}
 eval $1 / $2
-#
+
+binary-expr-op Mask
+op-type I
+vector
+### Note that this first "eval" is a dummy - we'll never generate code
+### that uses it because "Mask" expressions don't have LHS operands of
+### type "int". We could omit this if we modified Gen-ZAM to understand
+### that an op-type of 'X' for a binary-expr-op means "skip the usual case
+### of two operands of the same type".
+eval $1 / $2
 eval-mixed A I	auto mask = static_cast<uint32_t>($2);
 		auto a = $1->AsAddr();
 		if ( a.GetFamily() == IPv4 && mask > 32 )