Netmask Question

[ZaneZ]

I'm trying to learn about routing, so I'm messing around with some older Cisco routers and switches. Here's something that I've never understood and would love for somebody to explain. Lets say that I have two routers and a 192.168.0.0/24 network, a 192.168.1.0/24 network, and a 192.168.2.0/24 network. Lets say that all these are connected to a R2, and the that link between router R1 and R2 is 10.10.0.0/24.

If I'm making static entries in R1 for the three 192.168 networks connected to R2, do I have to make three separate entries, or can I have one entry where I have 192.168.0.0/16. That's always confused me. Can I just decrease the netmask whenever I want to include multiple networks that I have a larger netmask?

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[Joseph W. Doherty · Accepted solution]

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Posting

Yes you often can "roll-up" smaller subnets into a larger subnet for routing purposes.  This is called route aggregation or route summarization.  When dealing with a large number of networks, it's often the key for efficiency.

In your example, although you could roll-up your three consecutive /24s into the /16, often you would roll-up to less broad subnet.  For example, you could roll up your first two subnets into 192.168.0.0/23 and and keep 192.168.2.0/24.  (I.e. reduce your three route statements to two.)

Or you might roll-up all three of your /24s into 192.168.0.0/22.  This though also includes subnet 192.168.3.0/24.  So the question is, if somehow a packet for that subnet hits your static route, do you want it also to go to R2?  (Similar question using the /16, although then subnets 192.168.3.0/24 to 192.168.255.0/24 are also routed to R2.)

[ZaneZ]

Thanks a ton. So I guess this is like routing basics, can there be a 192.168.0.0/16 network, and then also be a 192.168.0.0/24 network. Or is the /24 network considered to be a part of the /16 network? That's always confused me.

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[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

No, there shouldn't be both an 192.168.0.0/16 and 192.168.0.0/24 network, but there may be an 192.168.0.0/16 aggregate over a 192.168.0.0/24 network.

What's confusing, you sometimes can't tell whether a route represents an aggregate or a network.

Real networks will eventually map into L3 interface IP for that network.

For example, I might have a router with two gateway interfaces of 192.168.0.1/24 and 192.168.1.254/24, but for routing purposes, instead of routing to this device for each of these individual networks (i.e. routes for 192.168.0.0/24 and 192.168.1.0/24), I might have a route of 192.168.0.0/23.

[ZaneZ]

Thanks. That makes pretty good sense haha

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