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TOPIC: IP Subnet Zero - valid in CCNA exam?

IP Subnet Zero - valid in CCNA exam? 11 years 10 months ago #5704

  • gl0bal
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Can someone help clarify for me whether Cisco considers using IP Subnet Zero valid or invalid (and thus getting the answer wrong) :roll: when sitting the CCNA exam.

I have studied the subnetting pages of this website www.firewall.cx/ip-subnetting-mask-bits.php and also studied the CCNA study Guide (4th ED) by Todd Lammie.

Firewall's articles talk as though its a given that you should use IP SUbnet Zero yet Todd Lammie's book says that IP Subnet Zero "can" be used in the real world yet for CCNA exam purposes you should not and I cannot find a definitive answer on the Cisco website.

Will I be penalised for using IP SUbnet Zero in the CCNA exam?

Thanks
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Re: IP Subnet Zero - valid in CCNA exam? 11 years 10 months ago #5705

  • nske
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I don't see why IP subnet Zero (0.0.0.0) would ever be considered invalid. Mathematically it represents an address space that covers every possible IP address and it is used all the time to define the default route/rule to follow for any IP address that does not much any specific route/rule.
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Re: IP Subnet Zero - valid in CCNA exam? 11 years 10 months ago #5716

  • gl0bal
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Perhaps I should clarify.
When I say subnet zero I am talking for example about 192.168.1.0/24 or 10.1.1.0/24 or any other IP address range with any mask that means a valid IP range could start at the 0 range. I understand Cisco frown on this because it means potential confusion with the default route.

Thanks for your comments so far.
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Re: IP Subnet Zero - valid in CCNA exam? 11 years 10 months ago #5717

  • nske
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ah now I see, sorry for the misunderstanding. I can't help you as I don't know the definite answer about cisco.

Still, in subnet /24, x.x.x.0 wouldn't be a valid IP address. In other custom subnets it could be, at least in theory. But my impression is that in practice the last (and the first of course) octets should never be 0, which is not a serious limit anyway.
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IP Subnet- Zero 11 years 10 months ago #6020

  • dpinard
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I found this article on IP subnet-zero which may provide an explanation to your question. Those subnets can easily be used providing the IP subnet-zero command is enabled on your router. As for CISCO CCNA, it's been a while since I wrote the exam but I believe they discourage the use of those subnets (although I'm not 100% sure).

www.faqs.org/faqs/cisco-networking-faq/section-39.html

How come my cisco router doesn't accept an address like:
"ip address 192.111.107.1 255.255.255.240"
or "ip address 171.69.0.1 255.255.0.0"

When "subnetting" of IP networks was first sanctioned by the IETF, the first and last subnets (the all zeros subnet and all ones subnet) were reserved for rather obscure uses and because of the confusion that would be caused with routing protocols that don't carry net mask information. It was technically illegal to place hosts or routers on those two subnets.

Several hosts and most other vendor's router products have problems operating
with the reserved subnets, so their use is discouraged. However, in 1995, the IETF removed the restrictions on the use of these reserved subnets as part of the classless routing effort.

If you would like to use the reserved subnets, simply add the line
"ip subnet-zero" to your cisco configuration.

You might consider adding "ip subnet-zero" to all your configurations as a matter of course, to avoid being bitten by this in the future.
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Re: IP Subnet Zero - valid in CCNA exam? 11 years 10 months ago #6023

  • gl0bal
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Thanks dpinard

I'm going in with the opinion that subnet-zero is discouraged in the exam but is used in the "real world".

Cheers for the link as well.
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