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diff --git a/src/lib/dhcpsrv/subnet_id.h b/src/lib/dhcpsrv/subnet_id.h
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+++ b/src/lib/dhcpsrv/subnet_id.h
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+// Copyright (C) 2014-2022 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/.
+
+#ifndef SUBNET_ID_H
+#define SUBNET_ID_H
+
+#include <exceptions/exceptions.h>
+#include <stdint.h>
+#include <typeinfo>
+#include <limits>
+#include <set>
+
+namespace isc {
+namespace dhcp {
+
+/// @brief Defines unique IPv4 or IPv6 subnet identifier.
+///
+/// Each subnet for which the DHCP service has been configured is identified
+/// by the unique value called subnet id. Right now it is represented as
+/// a simple unsigned integer. In the future it may be extended to more complex
+/// type.
+typedef uint32_t SubnetID;
+
+/// @brief Special value is used for storing/recognizing global host reservations.
+static const SubnetID SUBNET_ID_GLOBAL = 0;
+/// @brief The largest valid value for auto-generated subnet IDs.
+static const SubnetID SUBNET_ID_MAX = std::numeric_limits<uint32_t>::max()-1;
+/// @brief Special value used to signify that a SubnetID is "not set"
+static const SubnetID SUBNET_ID_UNUSED = std::numeric_limits<uint32_t>::max();
+
+/// @brief Exception thrown upon attempt to add subnet with an ID that belongs
+/// to the subnet that already exists.
+class DuplicateSubnetID : public Exception {
+public:
+    DuplicateSubnetID(const char* file, size_t line, const char* what) :
+        isc::Exception(file, line, what) { };
+};
+
+/// @brief Ordered list aka set of subnetIDs.
+typedef std::set<dhcp::SubnetID> SubnetIDSet;
+
+}
+}
+
+#endif // SUBNET_ID_H