    Network Working Group                                       D. McPherson
    Request for Comments: 3069                          Amber Networks, Inc.
    Category: Informational                                         B. Dykes
                                                             Onesecure, Inc.
                                                               February 2001

              VLAN Aggregation for Efficient IP Address Allocation

    Status of this Memo

       This memo provides information for the Internet community.  It does
       not specify an Internet standard of any kind.  Distribution of this
       memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2001).  All Rights Reserved.

    Abstract

       This document introduces the concept of Virtual Local Area Network
       (VLAN) aggregation as it relates to IPv4 address allocation.  A
       mechanism is described by which hosts that reside in the same
       physical switched infrastructure, but separate virtual broadcast
       domains, are addressed from the same IPv4 subnet and share a common
       default gateway IP address, thereby removing the requirement of a
       dedicated IP subnet for each virtual Local Area Network (LAN) or
       Metropolitan Area Network (MAN).

       Employing such a mechanism significantly decreases IPv4 address
       consumption in virtual LANs and MANs.  It may also ease
       administration of IPv4 addresses within the network.

    1. Introduction

       The VLAN [802.1Q] aggregation technique described in this document
       provides a mechanism by which hosts that reside within the same
       physical switched infrastructure, but separate virtual broadcast
       domains, may be addressed from the same IPv4 subnet and may share a
       common default gateway IPv4 address.

       Such a mechanism provides several advantages over traditional IPv4
       addressing architectures employed in large switched LANs today.  The
       primary advantage, that of IPv4 address space conservation, can be
       realized when considering the diagram in Figure 1:

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    RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001

       Figure 1:

        +------+    +------+    +------+    +------+    +------+
        |      |    |      |    |      |    |      |    |      |
        | A.1  |    | A.2  |    | B.1  |    | C.1  |    | B.2  |
        |      |    |      |    |      |    |      |    |      |
        +------+    +------+    +------+    +------+    +------+
            \          |           |           |            /
              \        |           |           |          /
                \ +-----------------------------------+ /
                  |                                   |
                  |          Ethernet Switch(es)      |
                  |                                   |
                  +-----------------------------------+
                                   |
                                   |
                              +--------+
                              |        |
                              | Router |
                              |        |
                              +--------+

       In the Figure 1 hosts A.1 and A.2 belong to customer A, VLAN A.
       Hosts B.1 and B.2 belong to customer B, VLAN B.  Host C.1 belongs to
       customer C and resides in it's own virtual LAN, VLAN C.

       Traditionally, an IP subnet would be allocated for each customer,
       based on initial IP requirements for address space utilization, as
       well as on projections of future utilization.  For example, a scheme
       such as that illustrated in Table 1 may be used.

       Table 1:
                                    Gateway     Usable   Customer
         Customer   IP Subnet       Address     Hosts    Hosts
         ========   ============    =======     ======   ========
         A          1.1.1.0/28      1.1.1.1     14       13
         B          1.1.1.16/29     1.1.1.17    6        5
         C          1.1.1.24/30     1.1.1.25    2        1

       Customer A's initial deployment consists of 2 hosts, though they
       project growth of up to 10 hosts.  As a result, they're allocated the
       IP subnet 1.1.1.0/28 which provides 16 IP addresses.  The first IP
       address, 1.1.1.0, represents the subnetwork number.  The last IP
       address, 1.1.1.15, represents the directed broadcast address.  The
       first usable address of the subnet, 1.1.1.1, is assigned to the
       router and serves as the default gateway IP address for the subnet.
       The customer is left 13 IP addresses, even though their requirement
       was only for 10 IP addresses.

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    RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001

       Customer B's initial deployment consists of 2 hosts, though they
       project growth of up to 5 hosts.  As a result, they're allocated the
       IP subnet 1.1.1.16/29 which provides 8 IP addresses.  The first IP
       address, 1.1.1.16, represents the subnetwork number.  The last IP
       address, 1.1.1.23, represents the directed broadcast address.  The
       first usable address of the subnet, 1.1.1.17, is assigned to the
       router and serves as the default gateway IP address for the subnet.
       The customer is left 5 with IP addresses.

       Customer C's initial deployment consists of 1 host, and they have no
       plans of deploying additional hosts.  As a result, they're allocated
       the IP subnet 1.1.1.24/30 which provides 4 IP addresses.  The first
       IP address, 1.1.1.24, represents the subnetwork number.  The last IP
       address, 1.1.1.27, represents the directed broadcast address.  The
       first usable address of the subnet, 1.1.1.25, is assigned to the
       router and serves as the default gateway IP address for the subnet.
       The customer is left 1 IP address.

       The sum of address requirements for all three customers is 16.  The
       most optimal address allocation scheme here requires 28 IP addresses.

       Now, if customer A only grows to use 3 of his available address, the
       additional IP addresses can't be used for other customers.

       Also, assume customer C determines the need to deploy one additional
       host, and as such, requires one additional IP address.  Because all
       of the addresses within the existing IP subnet 1.1.1.24/30 are used,
       and the following address space has been allocated to other
       customers, a new subnet is required.  Ideally, the customer would be
       allocated a /29 and renumber host C.1 into the new subnet.  However,
       the customer is of the opinion that renumbering is not a viable
       option.  As such, another IP subnet is allocated to the customer,
       this time perhaps a /29, providing two additional addresses for
       future use.

       As you can see, the number of IP addresses consumed by the subnetwork
       number, directed broadcast address, and a unique gateway address for
       each subnet is quite significant.  Also, the inherent constraints of
       the addressing architecture significantly reduce flexibility.

    2. Discussion

       If within the switched environment, on the routed side of the
       network, we introduce the notion of sub-VLANs and super-VLANs, a much
       more optimal approach to IP addressing can be realized.

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    RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001

       Essentially, what occurs is that each sub-VLAN (customer) remains
       within a separate broadcast domain.  One or more sub-VLANs belong to
       a super-VLAN, and utilize the default gateway IP address of the
       super-VLAN.  Hosts within the sub-VLANs are numbered out of IP
       subnets associated with the super-VLAN, and their IP subnet masking
       information reflects that of the super-VLAN subnet.

       If desired, the super-VLAN router performs functions similar to Proxy
       ARP to enable communication between hosts that are members of
       different sub-VLANs.

       This model results in a much more efficient address allocation
       architecture.  It also provides network operators with a mechanism to
       provide standard default gateway address assignments.

       Let's again consider Figure 1, now utilizing the super-VLAN sub-VLAN
       model.  Table 2 provides the new addressing model.

       Table 2:
                                    Gateway     Usable   Customer
         Customer   IP Subnet       Address     Hosts    Hosts
         ========   ============    =======     ======   ========
         A          1.1.1.0/24      1.1.1.1     10       .2-.11
         B          1.1.1.0/24      1.1.1.1     5        .12-.16
         C          1.1.1.0/24      1.1.1.1     1        .17

       Customer A's initial deployment consists of 2 hosts, though they
       project growth of up to 10 hosts.  As a result, they're allocated the
       IP address range 1.1.1.2 - 1.1.1.11.  The gateway address for the
       customer is 1.1.1.1, the subnet is 1.1.1.0/24.

       Customer B's initial deployment consists of 2 hosts, though they
       project growth of up to 5 hosts.  As a result, they're allocated the
       IP address range 1.1.1.12 - 1.1.1.16.  The gateway address for the
       customer is 1.1.1.1, the subnet is 1.1.1.0/24.

       Customer C's initial deployment consists of 1 host, and they have no
       plans of deploying additional hosts.  As a result, they're allocated
       the IP address 1.1.1.17.  The gateway address for the customer is
       1.1.1.1, the subnet is 1.1.1.0/24.

       The sum of address requirements for all three customers is 16.  As a
       result, only 16 addresses are allocated within the subnet.  These 16
       addresses, combined with the global default gateway address of
       1.1.1.1, as well as the subnetwork number of 1.1.1.0 and directed
       broadcast of 1.1.1.255, result in a total of 19 addresses used.  This
       leaves 236 additional usable hosts address with the IP subnet.

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    RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001

       Now, if customer A only grows to use 3 of his available addresses,
       the additional IP addresses can be used for other customers.

       Also, assume customer C determines the need to deploy one additional
       host, and as such, requires one additional IP address.  The customer
       is simply allocated the next available IP address within the subnet,
       their default gateway remains the same.

       The benefits of such a model are obvious, especially when employed in
       large LANs or MANs.

    3. Use of Directed Broadcasts

       This specification provides no support for directed broadcasts.
       Specifically, the <net, subnet, -1> directed broadcast address can
       only apply to one of the Layer 2 broadcast domains.

       Though use of directed broadcast is frowned upon in the Internet
       today, there remain a number of applications, primarily in the
       enterprise arena, that continue to use them.  As such, care should be
       taken to understand the implications of using these applications in
       conjunction with the addressing model outlined in this specification.

    4. Multicast Considerations

       It is assumed that the Layer 2 multicast domain will be the same as
       the Layer 2 broadcast domain (i.e., VLAN).  As such, this means that
       for an IP multicast packet to reach all potential receivers in the IP
       subnet the multicast router(s) attached to the IP subnet need to
       employ something akin to IP host routes for the sender in order for
       the Reverse Path Forwarding check to work.

    5. Deployment Considerations

       Extreme Networks has a working implementation of this model that has
       been deployed in service provider data center environments for over a
       year now.  Other vendors are rumored to be developing similar
       functionality.

    6. Security Considerations

       One obvious issue that does arise with this model is the
       vulnerabilities created by permitting arbitrary allocation of
       addresses across disparate broadcast domains.  It is advised that
       address space ranges be made sticky.  That is, when an address or
       range of addresses is allocated to a given sub-VLAN, reception of IP

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    RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001

       or ARP packets on a sub-VLAN with a source IP address that isn't
       allocated to the sub-VLAN should be discarded, and perhaps trigger a
       logging message or other administrative event.

       Implementation details are intentionally omitted as all functions in
       this document should remain local to the super-VLAN router.  As such,
       no interoperability issues with existing protocols should result.

    7. Acknowledgements

       Thanks to Mike Hollyman and Erik Nordmark for their feedback.

    8. References

       [802.1Q]  IEEE 802.1Q, "Virtual LANs".

    9. Authors' Addresses

       Danny McPherson
       Amber Networks, Inc.
       48664 Milmont Drive
       Fremont, CA  94538

       EMail: danny@ambernetworks.com

       Barry Dykes
       OneSecure, Inc.
       2000 S. Colorado Blvd Suite 2-1100
       Denver, CO.  80222

       EMail:  bdykes@onesecure.com

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    RFC 3069       VLAN Aggregation for IP Address Allocation  February 2001

    10.  Full Copyright Statement

       Copyright (C) The Internet Society (2001).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
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       The limited permissions granted above are perpetual and will not be
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       This document and the information contained herein is provided on an
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    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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