Can I avoid IP range overlap?

RouterOS General
1

Hi, pardon my lack of proper technical language, I’ll try to be as clear as possible.

I have an home network (Mikrotik Router A, color red) with ip 192.168.1.1
This is also a VPN server (SSTP) on 20.20.20.1

I want to deploy a second Mikrotik Router B color green, in another location.
In this location internet is provided by a generic LTE modem, with DHCP server in class 192.168.1.1/24. I can’t change this.
The B router will be connected to A router using SSTP tunnel, so B will be an SSTP client.

The problem:
Since Mikrotik B must acquire a wan ip in class 192.168.1.x (eg. 192.168.1.50) in order to have internet, could this pose a problem in the rest of the configuration, considering that 192.168.1.x is also the class of my Mikrotik A router?

this is the network schema, I hope it’s clear what my question is…

2

Mikrotik has many options here … . Multiple LAN with the same IP address range are usually not very simple routing connections.

Maybe you should look at the following …

  1. The ISP is giving you one subnet 192.168.1.0/24, which cannot be changed. But you can have multiple subnets in that one ISP subnet, that do not overlap in range.
    That is 2 separate 192.168.1.x/25 or 4 separate 192.168.1.y/26 subnets. You might need to use proxy-ARP on Router B to reach the Router A subnet

  2. Another way is to to masquerade the 2 identical subnets, or at least one of them and NAT them to separate subnets. See “netmap” in https://wiki.mikrotik.com/wiki/Manual:IP/Firewall/NAT
    And for how to use it, see: http://forum.mikrotik.com/t/route-to-multiple-remote-locations-with-same-lan-subnet-network/130313/1

  3. Also SSTP (just as PPTP and L2TP) has a BCP option, which can link the LAN of Router A and Router B at network level 2, so it is just one LAN for level 3 subnet.
    MT Documentation is for PPTP only, but it is the same for SSTP and L2TP. https://wiki.mikrotik.com/wiki/Manual:BCP_bridging_(PPP_tunnel_bridging)
    Be aware that RST must be enabled on the bridges for the BCP to be activated.

3

You can’t change the network on the LTE modem, but you can on the router A. I recommend setting the low-use band to .200-250
such as 192.168.235.0/24 this will avoid similar problems in the future.

4

What subnet is used between ISP modem vs router A or B and the subnets used behind router A or B, are two different things.

A lot of ISPs use 192.168.0.0/24 as subnet or 192.168.1.0/24.
Just make sure it is different behind your router and there will be no problem.

5

Yes @holvoetn, having different subnets behind Router A than between ISP and Router B would be an advantage for the LAN network. If OP is ready to change the subnet behind Router A, as a first requirement.

Secondly I have seen more ISP modem/routers in our country that allowed to select the X in the subnet range 192.168.X.0/24 , than that we had access to add eg Router B as a LAN Router for deeper and different private LAN subnets.

Normally this is remedied by some NAT or masquerade on Router B, towards the ISP modem/router, just to capture all that traffic ( mostly responses) coming from Internet.
Client devices can be connected to the ISP modem/router, but the major part is usually in the Router A LAN, or in the Router B LAN.
How LAN A and LAN B is interconnected can be in different ways, depending on needs and wishes.
With NAT and masquerade, the clients on the ISP/modem LAN will be reachable from LAN A and from LAN B, but will have problems in initiating connection to devices on LAN A or LAN B.


PS: This PS might be confusing, and probably not what is wanted …

One can even connect the "ISP modem/router "network, to the Router A LAN network, at layer 2, with the SSTP/BCP setup. But before you try, think again and again. This is bypassing Router B and Router A ‘WAN to LAN protections’. LAN A and ISP modem network, are then just 1 network.