How can I have a single /24 private IP address space that spans two locations across a VPN?

aka VPLS


We have a /24: that has workstations and servers on it. Servers are .5 through .30, workstations are .50-.254.

  • Currently, all this is at a single physical location.
  • The servers are all VMWare instances on a single VMWare host.
  • We currently use Sonicwall NSA 3600 and similar models

We need to move the servers to a data center (to reclaim the office space, etc), and, for a number of reasons, need to keep the networking as is. (There is extensive configuration at app server and client level giving specific rights to specific IP address, blocking access by IP, and putting different types of users in different IP address blocks.)

When I have done VPN configs in the past, we always had a different subnet at each location. This would break many layers of app setup, so we are looking to avoid it. We need the same subnet ( to span two locations, and for the servers to see the clients properly (e.g. the client IP is represented to the server as it is now, when everything is in one physical location).

I have heard that MPLS and/or L2 servers from a telco may be able to solve for this, but I do not know exactly how. We would prefer to implement this in our own networking gear (to give us flexibility RE location, e.g. not to be completely dependant on telco), but need to understand the options.

Note: - We understand that L2 bridging is not ideal from some perspectives - It is, however, the right thing for this application - We know about using NAT to route a "local" address across to another subnet. No need to explain that. This thread is "how to span a /24 subnet across two location"

a) Can this be done at all?

b) We know that telco has services like this. We want to know: Can we do it in our own gear? (We currently use the Sonicwall NSA 3600, but will migrate off if we need to. What do we need to have, spec wise, in gear to do that?)

c) Suggested "closest options" to this? (e.g. if our dream of VPLS in our own gear is not possible, is there another option between that and telco VPLS that solves in this way?)

  • First, classful addressing is dead. It was killed in the last century by VLSM and CIDR (see RFCs 1518 and 1519). Any reference to a network class is purely historical, and you should not use classes today. All modern networking is classless You need to provide more information. We don't know how things are connected, or what equipment models you do have. Also, product or resource recommendations are explicitly off-topic here.
    – Ron Maupin
    Oct 2, 2016 at 19:57
  • There are ways to extend a layer-2 domain across a WAN, but this is less than ideal. Any layer-2 problems (e.g. broadcast storms, etc.) will be magnified by the WAN latency, and you will now affect two sites, rather than just one. The real answer is to readdress the servers, and use domain names rather than IP addresses for specific servers.
    – Ron Maupin
    Oct 2, 2016 at 20:00
  • @RonMaupin Ron, I updated the OP and tried to address the points you raise. Thank you! Oct 2, 2016 at 20:17
  • What do you mean by, "c) Suggested "closest options" to this?"
    – Ron Maupin
    Oct 2, 2016 at 20:26
  • 1
    There is a good discussion in this thread: networkengineering.stackexchange.com/questions/208/… Jun 6, 2018 at 0:06

3 Answers 3


Understanding and Configuring VLAN Routing and Bridging on a Router Using the IRB Feature

Background Information

In order for a VLAN to span a router, the router must be capable of forwarding frames from one interface to another, while maintaining the VLAN header. If the router is configured for routing a Layer 3 (network layer) protocol, it will terminate the VLAN and MAC layers at the interface a frame arrives on. The MAC layer header can be maintained if the router is bridging the network layer protocol. However, regular bridging still terminates the VLAN header.

Using the IRB feature in Cisco IOS® Release 11.2 or greater, a router can be configured for routing and bridging the same network layer protocol on the same interface. This allows the VLAN header to be maintained on a frame while it transits a router from one interface to another. IRB provides the ability to route between a bridged domain and a routed domain with Bridge Group Virtual Interface (BVI). The BVI is a virtual interface within the router that acts like a normal routed interface that does not support bridging, but represents the comparable bridge group to routed interfaces within the router. The interface number of the BVI is the number of the bridge group that the virtual interface represents. The number is the link between the BVI and the bridge group.


a) Can this be done at all?

Yes, it can.

b) Can we do it in our own gear? (We currently use the Sonicwall NSA 3600, but will migrate off if we need to. What do we need to have, spec wise, in gear to do that?)

Yes, you can do this with your firewalls; they support layer-2 tunneling.

c) Suggested "closest options" to this?

I'm not sure what you mean by this question. You can do what you want with your existing equipment, and there are many other options to do this, but product or resource recommendations are explicitly off-topic here.

Of note is that you want flexibility, but you have painted yourselves into a corner by specifically using IP addresses, removing all your flexibility. Your applications should be using domain names rather than specific server IP addresses. If you correct that problem, you will regain your flexibility. This is the point in time which you should bite the bullet and fix things to the way they should be.

  • Thank you! We will do a bench test with layer-2 tunneling. Neither I nor our sonicwall consultant have used that for site to site before. Oct 2, 2016 at 22:35

Of course it's possible.

Openvpn in bridge mode with Linux boxes as the terminations is one soloution. There are probabll more enterprisy ones too.

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