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Community Member

Subnet zero and Broadcast subnet on a /25 mask

Just going through some test questions before my CCENT exam on Friday and I'm thinking about the following question.

Let's say you have a topology with 6 subnets, 3 on the serial links and 3 on the Lan's. RIP2 has been configured as the routing protocol but with no ip subnet-zero on all routers. Lets say one of the serial links was using a /25, implying the block size is 128. Wouldn't both sides of this subnet (IP addresses) be un-usable because the lower side falls under the subnet zero and the other side is the broadcast subnet?

Cheers guys!

8 REPLIES
Community Member

Re: Subnet zero and Broadcast subnet on a /25 mask

Just thought, i've got a lab here, i'll just create the scenario!! Still open to the replies tho! Cheers!

Hall of Fame Super Gold

Re: Subnet zero and Broadcast subnet on a /25 mask

Garry

You start your question mentioning 6 subnets, then you describe one of them as /25 and ask about its limitations. But you never tell us what address space you are dealing with. The question sort of implies working with a /24 network, but you never do specify what context you are dealing with. If the network were a /16 there would be no issues with the /25 subnet.

To respond to the question that I think you intended to ask: in a /24 (class C) network if you configure /25 subnets and if you configure no ip subnet-zero, then the first subnet is reserved and you can not use it. There is no problem with using the second subnet. (subnet zero can be reserved if you configure no ip subnet-zero, but nothing reserver the second /25 - which some people describe as the broadcast subnet - but the broadcast subnet is not reserved).

Lab it up. Check it out. And let us know what you find.

HTH

Rick

Community Member

Re: Subnet zero and Broadcast subnet on a /25 mask

Hi Rick,

Thanks for the reply!

My idea came from a question in the CCENT boson exam, it was directed at subnetting.

I did lab it up last night, i used a class C network - 192.168.1.0 with a prefix /25 subnet (block size of 128) on the serial back to back link.

I assigned R1 s0 192.168.1.1 and assigned R2 s0 192.168.1.2, default encapsulation was used as the datelink protocol, both routers are connecting over a DCE - DTE back to back serial cable. I used the no ip subnet-zero on both routers and both interfaces on both routers were in an up/up status. I was able to ping the Ethernet port on R2 from a PC connected to the Ethernet port on R1. So this tells me, even though the no ip subnet-zero is applied on both routers the subnet-zero address space can still be used, or i've done something wrong along the line... :-)

Just to note, you did state the broadcast subnet is not reservered, i think the exam expects you to know when to -2 on the subnets just like you -2 on host addresses. I will check this out.

Hall of Fame Super Gold

Re: Subnet zero and Broadcast subnet on a /25 mask

Garry

Let me suggest a refinement of your test. In your test the addresses worked because you configured the addresses first and then configured no ip subnet-zero. Let me suggest that you remove the ip addresses with the /25 in the reserved part of the subnet. Make sure that no ip subnet-zero is configured first and then attempt to configure the addresses with the /25 mask. At that point the subnet should be reserved and you should get an error message complaining about bad mask.

Give it a try and let us know what happens.

Just a note: you never go wrong doing -2 for subnets, but it is not required. The broadcast subnet is always available (and always has been).

HTH

Rick

Hall of Fame Super Bronze

Re: Subnet zero and Broadcast subnet on a /25 mask

I did lab it up last night, i used a class C network - 192.168.1.0 with a prefix /25 subnet (block size of 128) on the serial back to back link.

Subnet Zero would be 192.168.0.0/25, not 192.168.1.0/25 as the 0 on the latter subnet is actually a host, not a subnet.

Try 192.168.0.0/25 with no ip subnet-zero and you will see the difference :)

HTH,

__

Edison.

Hall of Fame Super Gold

Re: Subnet zero and Broadcast subnet on a /25 mask

Edison

I am quite puzzled by your response. 192.168.0.0/25 is subnet zero for one class C network. And 192.168.1.0/25 is subnet zero for a different class C network. He should get exactly the same behavior for 192.168.0.1/25 and for 192.168.1.1/25. (if no ip subnet-zero is configured both addresses should generate an error about invalid mask)

HTH

Rick

Hall of Fame Super Bronze

Re: Subnet zero and Broadcast subnet on a /25 mask

Hi Rick,

You are correct:

Rack1R1(config)#no ip subnet-zero

Rack1R1(config)#int f0/0

Rack1R1(config-if)#ip address 192.168.0.1 255.255.255.128

Bad mask /25 for address 192.168.0.1

Rack1R1(config-if)#ip address 192.168.1.1 255.255.255.128

Bad mask /25 for address 192.168.1.1

Rack1R1(config)#ip subnet-zero

Rack1R1(config)#int f0/0

Rack1R1(config-if)#ip address 192.168.1.1 255.255.255.128

Rack1R1(config-if)#

__

Edison.

Community Member

Re: Subnet zero and Broadcast subnet on a /25 mask

Rick, Edison, apologies for the late reply on this thread, i was busy getting some last minute revision in and passed my CCENT yesterday :-)

Thanks for the replies, i did lab it up and got the expected results!

Cheers guys

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