Ms press Subnetting question, page 2-55

In the process of getting my head around subnetting, its coming along slowly and is proving rather difficult. Just a quick question:

On page 2-55, the question : What is the configuration error?
The router has an external /24 subnet mask, and has 2 internal subnets, both with /21 subnet masks.

I thought that internal subnets could only have a value that was greater than the external value,
i.e in this case, anything from the 8 bits left for the host ID.

Im probably missing some really easy and simplistic, so apologies in advance

Cheers

Anth

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Comments

royal

Welcome to the site.

Any subnet can be used really as long as they own the address space. If ICANN assigns them /24 for public use, then that company can use any subnet /range that's lower due to the fact they own all the IPs within that /24 range. Technically, that's called using VLSM, Virtual Length Subnet Mask.

So in essence (fake scenario), ICANN gives me 200.200.200.0/24. I now have control over all networks within 200.200.200.0. That means, that even with a /23, that is still within the /24 of networks ICANN gave me. I'm just further segmenting my network to prevent broadcasts that aren't needed; or for some other reason I'd want to segment.

If the /24 is for public use, they can still use any other range they want internally if they are using private IP addressing.

sprkymrk

If you are assigned /24, but use a /21 mask, you are actually including addresses outside your allowed range. You can go the other way though, and split your network into pieces like:

200.200.200.0/24 = 200.200.200.1- 255
200.200.200.0/25 = 200.200.200.1 - 126
200.200.200.0/26 = 200.200.200.1 - 62

etc., but

200.200.200.0/21 = 200.200.200.1-255 and 200.200.201.1-255 and 200.200.202.1-255 etc, up to 200.200.207.1-255

royal

Er, ya. Mark is right. I meant going up /24 /25 /26 is fine since you own those IPs, not the other way around.

But my point still stands with using /21 on the inside, it'd be fine if you're using private IPs. But for your public IPs, you'd only be able to go up /24 /25 /26. That's VLSM.

liddane

Thanks for the info, its starting to become a little more clear now. The Ms press book is good but I think I may need a broader range of resources. Can anyone recommend any good sites?

Thanks

Anth

dynamik

This is a really good resource: http://www.thebryantadvantage.com/TheBryantAdvantageMasterySeries.html

I don't remember how Cisco-centric the focus is, but it should help you answer anything on this exam. He's good about getting back to people. Ask him what he thinks about using it for 291.

rhelt100

liddane wrote:

Thanks for the info, its starting to become a little more clear now. The Ms press book is good but I think I may need a broader range of resources. Can anyone recommend any good sites?

Thanks

Anth

If you have the CBTNuggets or know somebody that does, the video on subnetting is spectacular. I learned from it and feel that I completely understand subnetting. In addition, a colleague of mine was studying for the windows 2000 equivalent of the 70-291 and was banging his head against the wall with subnetting. I loaned him my disk and told him to watch the subnetting video. Bingo! The next day he tells me it all makes sense now.