This is a Canonical Question about solving IPv4 subnet conflicts between a VPN client's local network and one across the VPN link from it.

After connecting to a remote location via OpenVPN, clients try to access a server on a network that exists on a subnet such as However, sometimes, the network on the client's LAN has the same subnet address: Clients are unable to connect to the remote server via typing in its IP because of this conflict. They are unable to even access the public internet while connected to the VPN.

The problem is that this subnet needs to be routed by the VPN, but it also needs to be routed as the client's LAN.

Does anyone know how to mitigate this issue? I have access to the OpenVPN server.

  • 3
    You can try setting a static route for the /32 address of the host you're trying to reach and use the VPN peer as your gateway and see what happens. Oct 26, 2013 at 23:33
  • if the vpn concentrator honors client routes, then your perimeter security may need some help. i get access to accounting lan, add route to engineering range, and i can then connect no problem. crappy firewalls like sonicwall do this
    – nandoP
    Oct 27, 2013 at 0:05
  • @SpacemanSpiff: While this could solve the issue on the client side, the server would still be unable to reply, because it would see the connection as coming from its own network, not from a VPN client.
    – Massimo
    Dec 24, 2015 at 22:10

9 Answers 9


It is possible to solve this using NAT; it's just not very elegant.

So under the assumption you couldn't solve this by having internal nets which have so uncommon network numbers as to never actually come into conflict, here's the principle:

As both the local and remote subnet have identical network numbers, traffic from your client will never realize it has to go through the tunnel gateway to reach its destination. And even if we imagine it could, the situation would be the same for the remote host as it is about to send an answer.

So stay with me and pretend that as of yet, there are no side issues as I write that for full connectivity, you would need to NAT both ends inside the tunnel so as to differentiate the hosts and allow for routing.

Making some nets up here:

  • Your office network uses
  • Your remote office uses
  • Your office network VPN gateway hides hosts behind the NATed network number
  • Your remote office network VPN gateway hides hosts behind the NATed network number

So inside the VPN tunnel, the office hosts are now 198.51.100.x and remote office hosts are 203.0.113.x. Let's furthermore pretend all hosts are mapped 1:1 in the NAT of their respective VPN gateways. An example:

  • Your office network host is statically mapped as in the office vpn gateway NAT
  • Your remote office network host is statically mapped as in the remote office vpn gateway NAT

So when host in the remote office wants to connect to the host with the same ip in the office network, it needs to do so using the address as destination. The following happens:

  • At the remote office, host is a remote destination reached through the VPN and routed there.
  • At the remote office, host is masqueraded as as the packet passes the NAT function.
  • At the office, host is translated to as the packet passes the NAT function.
  • At the office, return traffic to host goes through the same process in the reverse direction.

So whilst there is a solution, there are a number of issues which must be addressed for this to work in practice:

  • The masqueraded IP must be used for remote connectivity; DNS gets complex. This is because endpoints must have a unique IP address, as viewed from the connecting host.
  • A NAT function must be implemented both ends as part of the VPN solution.
  • Statically mapping hosts is a must for reachability from the other end.
  • If traffic is unidirectional, only the receiving end needs static mapping of all involved hosts; the client can get away with being dynamically NATed if desirable.
  • If traffic is bidirectional, both ends need static mapping of all involved hosts.
  • Internet connectivity must not be impaired regardless of split- or non-split VPN.
  • If you can't map 1-to-1 it gets messy; careful bookkeeping is a necessity.
  • Naturally one runs the risk of using NAT addresses which also turn out to be duplicates :-)

So solving this needs careful design. If your remote office really consists of road warriors you add a layer of problems in that:

  • they never know beforehand when they end up on overlapping net ids.
  • the remote office gateway NAT would need to be implemented on their laptops.
  • the office gateway would need two VPNs, one NAT-free and one NATed, to cover both scenarios. Otherwise, in the event someone were to pick one of the subnets you chose for the NAT method, things wouldn't work.

Depending on your VPN client you might be able to automatically select one VPN or the other depending on the network address of the local segment.

Observe that all mentioning of NAT in this context denotes a NAT function which so to speak takes place within the tunnel perspective. Processwise, the static NAT mapping must be done before the packet "enters" the tunnel, i.e. before it is encapsulated in the transport packet which is to take it across the internet to the other VPN gateway.

This means that one must not confuse the public ip addresses of the VPN gateways (and which in practice may also be NAT:ed, but then wholly outside the perspective of transport to the remote site through VPN) with the unique private addresses used as masquerades for the duplicate private addresses. If this abstraction is difficult to picture, an illustration of how NAT may be physically separated from the VPN gateway for this purpose is made here:
Using NAT in Overlapping Networks.

Condensing the same picture to a logical separation inside one machine, capable of performing both the NAT and VPN gateway functionality, is simply taking the same example one step further, but does place greater emphasis on the capabilities of the software at hand. Hacking it together with for example OpenVPN and iptables and posting the solution here would be a worthy challenge.

Softwarewise it certainly is possible:
PIX/ASA 7.x and Later: LAN-to-LAN IPsec VPN with Overlapping Networks Configuration Example
Configuring an IPSec Tunnel Between Routers with Duplicate LAN Subnets

The actual implementation therefore depends on a lot of factors, the operating systems involved, associated software and its possibilities not the least. But it certainly is doable. You would need to think and experiment a bit.

I learned this from Cisco as seen by the links.


If you need a temporary dirty workaround to a single or a handful known server ips, the simplest solution should be the static client side routing option.

In my case I added my desired destination server ( to my routing table on my linux client via:

route add dev tun0

Afterwards, remove this static route with the route delete command.

  • 2
    This is a perfect solution, and an even perfect timing! :)
    – Yuval A
    Feb 28, 2017 at 13:12
  • How long does this persist? Until you disconnect? Until reboot? Jul 31, 2017 at 23:23
  • 2
    On my linux system (xfce with ubuntu/mint) the settings is "lost" after a vpn disconnect and yes, after a reboot as well. You can verify if the setting is active with the route command (there will be an entry having the ip and the tun0 device usually at the bottom)
    – Aydin K.
    Aug 9, 2017 at 15:40
  • 3
    The OSX version of route takes the interface differently so instead of dev tun0 you need -interface tun0
    – Allison
    Jan 4, 2018 at 18:22

The Answer from Aydin K. is for linux. If your want the same funtionality for windows, you can type

route ADD <IP of tunnel adapter>


route ADD IF <interface id>

you can get the interface id with the command:

route print
  • Could some kind employee at Apple/Android please build in a feature to accomplish this, somewhere in the advanced VPN settings? Or otherwise add a toggle option to route all traffic via the VPN as a last resort... It would be allot cheaper than using a mobile network connection instead of some local WiFi. Feb 17 at 23:20
  • I couldn't get the second syntax to work. Help documentation suggests that the destination interface IP is always required. e.g. route add mask
    – MZB
    Feb 18 at 16:25

yup this is the worst. for me it happened all the time from hotel rooms, before it vpn admins realized they should use more obscure ip ranges. and are the worst. if you can help it never ip a wireless network like that.

so the answer is "fix" the wap to use a different internal network (ie and then give you a diff lease (ie ip in a range that can route back to corp vpn), or if you have no / cant get admin on wap, just go to starbucks. or 20 minutes of wardriving :)

if this is just in a lab setting, just use different ranges.

  • Dang really? There is no better option? Oct 27, 2013 at 8:05
  • 1
    not that i know of.... this has always been a problem... looks like someone down-voted my answer, but didnt actually suggest a solution.... ha kill the messenger!
    – nandoP
    Nov 11, 2013 at 3:33

I'm on a mac running El Capitan. While the suggestions above didn't work for me they led me to a working solution:

  1. before starting VPN, execute ifconfig
  2. start the VPN, do ifconfig and note which is the new interface. In my case it was ppp0 with an ip address of

    ppp0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1280
        inet --> netmask 0xffffff00
  3. type in:

    sudo route add

I tested first with a ping and then I proved it worked by accessing the git server.

When I tried using dev ppp0 for the end of the route command as mentioned above, it complained.

  • 2
    What is coming from in this exchange? Jul 31, 2017 at 23:24
  • The destination server that you're connecting to. This server resides in the same network as your VPN, not your local connection. Jul 8, 2018 at 20:55

I have a simple solution that I'm using at a co-working space that has a conflicting IP range (10.x)

I connected to the network with my mobile phone, then I shared the network connection via bluetooth with my laptop. I can now use the VPN for my remote employer.

I'm sure this will work just the same via USB if you need a faster connection.

  • 1
    Hey, that actually is a pretty clever solution. Dec 21, 2016 at 2:05

If you just need to hit a few one or two ip addresses, add route statement to your ovpn configuration file like this:



It will add a route for just those Ip's when you connect your vpn and remove it when the vpn it disconnected.

Worked for me on Windows anyway.


Just as a reminder: this whole issue is due to years of IPv4 address shortage, and extensive use of private IP range behind NAT to workaround this shortage !

The ideal and definitive solution to this issue is quite straightforward (albeit it can, and will, takes some time to be globally rolled-out) : IPv6...

In an IPv6 world, there's no public IP shortage (and there won't be, event in a few decade). So there no reason not to have a public IP on each and every device of every network. And if you need network isolation, keep filtering with a firewall, but without ugly NAT...


if you have same ip block both local and remote networks and want to access only remote block local && remote network address : following options for windows 10 worked for me

add these options to OpenVPN certificate file

route-metric 3

why? Real ethernet interface have metric 0

TAP vpn interface has metric 256

we must increase TAP interface's priority

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