Routing question on a new private line.

Answered Question
Sep 24th, 2008

I know this is a basic routing question, but I can't remember what I need to do for my specific situation. I have a new T1 to a branch location that is running on a 192.168.0.0/24 network and our local network is a 192.168.0.0/16. I'm having a hard time conceptualizing how traffic generated from our local net destined to a 192.168.0.0/24 is going to make it to the remote location. I'm not sure if this is considered an overlapping network and the key is in using NAT. Thanks in advance for the help.

I have this problem too.
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Correct Answer by Richard Burts about 8 years 2 months ago

Jerrod

This does not confuse the issue and does help to clarify what is going on. As long as there are not addresses assigned on your side in the range 192.168.0.1 through 192.168.0.254 then it can work to use 192.168.0.0/24 at the branch. It will work because examination of the routing table always looks for the longest match.

Before I explain further I would like to point out one thing. Consider the prefixes

192.168.0.0/16

192.168.0.0/24

we might look at them and say that they are both 192.168.0.0 and are the same. But to the router and its routing logic they are different because they have different masks and they identify different address blocks.

So to make an example if you do show ip route you might get output that says that 192.168.0.0/16 is reachable through on fastethernet0/0 and that 192.168.0.0/24 is reachable through on serial1/1. And if your user on the LAN of site A sends a packet to address 192.168.0.44 then the router will know that it matches the more specific route in the table and will forward out the serial interface. And if that user sends a packet to 192.168.3.2 then the router will know that it matches the less specific route in the table and will forward out the fastehternet interface.

For this to work both site A and the branch must have entries in their routing table for both the /16 and the /24 subnets. (and in this case you would not need address translation)

HTH

Rick

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satish_zanjurne Wed, 09/24/2008 - 20:51

Hi,

1.Advertise specific network on both local network interface & remote location network interface ( WAN ).

2.Disable auto-summary in whichever routing protocol you are running.

I am not sure, whether you will require NAT.

HTH..rate if helpful..

Richard Burts Thu, 09/25/2008 - 04:26

Jerrod

If I am understanding your post correctly the branch is configured with 192.168.0.0/24 as its LAN. And your site is configured with 192.168.0.0/16. So there is overlap in the addressing between the sites. It might help us if we had some more information about the network at your site. It would be especially good to know whether there are addresses in the 192.168.0.0/24 that are used at your site.

If the block of addresses in 192.168.0.0/24 is not really used at your site then the suggestion by Satish to advertise the more specific route from the branch could work. If the address block is used at your site then you will need to translate addresses to make this work.

HTH

Rick

jstewart33 Thu, 09/25/2008 - 06:36

Thank you for your help. Currently there aren't any IP addresses on our local LAN that are in the 192.168.0.0/16.

Satish, currently I'm only going to be using static routes, so would your solution still work in this scenario.

Richard Burts Thu, 09/25/2008 - 11:25

Jerrod

I am a bit confused. In your initial post you say that the network on your side is 192.168.0.0/16. Now you say that there are no addresses on your local LAN in 192.168.0.0/16. Can you help me understand what addresses are used where?

HTH

Rick

jstewart33 Thu, 09/25/2008 - 12:39

Sorry for the confusion. I will try to explain myself better. Site A uses a 192.168.0.0/16 for it's IP addresses. Site B (branch location) uses 192.168.0.0/24 for it's IP addresses. Now I took from your previous post that you were asking whether or not there were any IP addresses in use that ranged from 192.168.0.1-254 in Site A even though they had a default 16 mask. I verified that there weren't any 192.168.0.1-254 addresses in use in Site A, even though they would work perfectly (meaning that these indeed are valid, but not in use). So I believe that because we are running a 192.168.0.0/24 in Site B there would be the potential of overlapping networks. Is this correct or is my understanding of overlapping networks incorrect? Let's say that I have a static route in Site A router (ip route 192.168.0.0 255.255.255.0 10.2.5.1) which 10.2.5.1 is Site B WAN Interface. Now a client in Site A tries to ping a machine in Site B with an IP of 192.168.0.1. How will traffic be able to traverse the WAN and not just stay on the LAN in Site A. I hope this makes sense and I didn't just confuse the issue even more. Thanks for your diligence.

Correct Answer
Richard Burts Thu, 09/25/2008 - 13:09

Jerrod

This does not confuse the issue and does help to clarify what is going on. As long as there are not addresses assigned on your side in the range 192.168.0.1 through 192.168.0.254 then it can work to use 192.168.0.0/24 at the branch. It will work because examination of the routing table always looks for the longest match.

Before I explain further I would like to point out one thing. Consider the prefixes

192.168.0.0/16

192.168.0.0/24

we might look at them and say that they are both 192.168.0.0 and are the same. But to the router and its routing logic they are different because they have different masks and they identify different address blocks.

So to make an example if you do show ip route you might get output that says that 192.168.0.0/16 is reachable through on fastethernet0/0 and that 192.168.0.0/24 is reachable through on serial1/1. And if your user on the LAN of site A sends a packet to address 192.168.0.44 then the router will know that it matches the more specific route in the table and will forward out the serial interface. And if that user sends a packet to 192.168.3.2 then the router will know that it matches the less specific route in the table and will forward out the fastehternet interface.

For this to work both site A and the branch must have entries in their routing table for both the /16 and the /24 subnets. (and in this case you would not need address translation)

HTH

Rick

jstewart33 Fri, 09/26/2008 - 05:51

Rick

I appreciate the help greatly. Thanks for helping me remember that sometimes you just have to slow down and remember the basics.

Jerrod

Richard Burts Fri, 09/26/2008 - 12:11

Jerrod

I am glad that my responses were informative and helpful. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that there were responses which did help resolve the question.

The forum is an excellent place to learn about Cisco networking. I encourage you to continue your participation in the forum.

HTH

Rick

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