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Managing two separate subnet with same class addresses

Posted: Wed May 29, 2019 10:12 am
by ik3umt
Is it possible to manage two LAN having each the same IP subnet both containing machines with same ip address ?

Image

I would say NO, but I'm not aware if Ros has some obscure feature......
I could use netmap to addressing two different subnet , but how to know on wich target machines ?

Re: Managing two separate subnet with same class addresses

Posted: Wed May 29, 2019 10:59 am
by BartoszP
Why?

Re: Managing two separate subnet with same class addresses

Posted: Wed May 29, 2019 11:08 am
by Sob
Why not? ;) It's of course better to not have something like this, but if it already happened and it's not possible to change it...

Netmap with some virtual subnets is good first step. Next one would be two routing tables, one for each subnet on different interfaces. And finally mangle rules in prerouting to choose the right routing table based on which virtual subnet is destination.

Re: Managing two separate subnet with same class addresses

Posted: Wed May 29, 2019 11:43 am
by ik3umt
Sob, How can I define a virtual subnet to the real one and routing through the right interface ?
I suppose each ethernet has not to be configured with an ip address...

Re: Managing two separate subnet with same class addresses

Posted: Wed May 29, 2019 5:03 pm
by anav
Why?
So Sob can show off his MT networking skills ;-)

Re: Managing two separate subnet with same class addresses

Posted: Thu May 30, 2019 12:00 am
by Sob
Simple example showing how to set up access from outside:

First some addresses (*):
/ip address
add address=192.168.0.1/24 interface=test1
add address=192.168.0.1/24 interface=test2
Routes in two different routing tables:
/ip route
add dst-address=192.168.0.0/24 gateway=test1 routing-mark=net1
add dst-address=192.168.0.0/24 gateway=test2 routing-mark=net2
Mangle rules (packets to virtual 192.168.91.x will get "net1" routing mark, and packets to virtual 192.168.92.x will get "net2" routing mark):
/ip firewall mangle
add action=mark-routing chain=prerouting dst-address=192.168.91.0/24 new-routing-mark=net1
add action=mark-routing chain=prerouting dst-address=192.168.92.0/24 new-routing-mark=net2
Map virtual networks to real ones:
/ip firewall nat
add action=netmap chain=dstnat dst-address=192.168.91.0/24 to-addresses=192.168.0.0/24
add action=netmap chain=dstnat dst-address=192.168.92.0/24 to-addresses=192.168.0.0/24
Small fix for when this router is not default gateway for both networks:
/ip firewall nat
add action=masquerade chain=srcnat out-interface=test1
add action=masquerade chain=srcnat out-interface=test2
(*) Same address and network on two interfaces like this is of course wrong. For this example, it could be maybe a little less wrong with /32 netmask.

This example also shows only the basic idea, the RB won't work as default gateway for both subnets. It could, but it would require additional connection marking. But since I don't know how exactly is everything connected, I won't bother with that now.

Re: Managing two separate subnet with same class addresses

Posted: Thu May 30, 2019 2:31 am
by ik3umt
I didn't know more interfaces can be configured with the same ip address and subnet , I thought this would mess up things.

It happens, rarely, but happens, you find a place having the same addressing i.e. for PC and for ip cameras or ip telephony, completely separated , each with its own switch (sometimes ip cameras network stand-alone without a router) and you're asked for managing both remtely......

Surely I'll give it a try !

Re: Managing two separate subnet with same class addresses

Posted: Thu May 30, 2019 3:48 am
by Sob
I thought it was clear that it does mess things up. :) The fact that to some extent it's possible to make it work doesn't change that it's wrong. If it's at all possible, the config should be changed into something clean, i.e. either join the networks correctly, or renumber some.

Re: Managing two separate subnet with same class addresses

Posted: Thu May 30, 2019 12:53 pm
by ik3umt
No, wait, I haven't undesrtood if two eth with the same ip address and class on the same routerboard is IMPOSSIBLE (don't work) or is WRONG (but works because of using interface names and routing marks).

Each LAN has it own gateway (not the routerboard).

Re: Managing two separate subnet with same class addresses

Posted: Thu May 30, 2019 2:36 pm
by savage
Only way to do this without messing things up, is to use a VRF