255.255.255.254??? what?? Subnet zero used..???

Ok I've seen this question too many times and i just dont understand it to be honest.

Using subnet zero, and a subnet mask of 255.255.255.254 which of the following are usuable host addresses?

192.168.1.0

192.168.2. 3

192.168.1.1

192.168.7.4

192.168.9.2

i am completley lost because from my understanding the /31 will give u no usuable host addresses...but with subnet zero i dont know....subnet zero provides for what? do you use the network address for a host address?? Someone please explain this to me if you can. I thought i understood this but when i came accross questions like this numerous times..i dont know i just don't get it anymore.

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Comments

Netwurk

First off, you are not going to get a question on /31 networks on the CCNA - so don't worry too much.

But as long as you asked....

Using /31 (255.255.255.254) gives you two addresses per subnet. You use them both as hosts. The drawback is that broadcasts are limited.

Here's the official Cisco take on it:

"RFC 3021 describes using 31-bit prefixes for point-to-point links. This leaves 1 bit for the host-id portion of the IP address. Normally a host-id of all zeros is used to represent the network or subnet, and a host-id of all ones is used to represent a directed broadcast. Using 31-Bit prefixes, the host-id of 0 represents one host, and a host-id of 1 represents the other host of a point-to-point link.

Local link (limited) broadcasts (255.255.255.255) can still be used with 31-bit prefixes. But directed broadcasts are not possible to a 31-bit prefix. This is not really a problem because most routing protocols use multicast, limited broadcasts, or unicasts."

The above is from http://www.cisco.com/warp/public/701/8.html#using31bit

DirtySouth

Subnet zero is kind of a pain in the arse because sometimes its assumed and other times its not. However, in this case apparently it is used.

What that means is, the address that is normally reserved only for network addresses can be used as a host address. So in this case 192.168.1.0 & 192.168.1.1 are both usable.

254 = 11111110

Using that binary mask, there are only two possibilities for addresses:
11111110 & 11111111.

The only two addresses that can fit into that range would be .0 & .1. Subnet zero allows you to use the first address (.0). Hopefully my logic hasn't confused you.

kenny504

Thanks alot guys...but here's the thing. I really don't want to scare anybody, but i could swear this is one of the questions i messed up on last week, when i sat the ICND!!. I am so sure. So i appreciate the help i think i have the concept now. But about this not being on the exam i wud'nt say that too loud. Hope i didnt say too much....

THanks again

Trailerisf

kenny504 wrote:

Thanks alot guys...but here's the thing. I really don't want to scare anybody, but i could swear this is one of the questions i messed up on last week, when i sat the ICND!!. I am so sure. So i appreciate the help i think i have the concept now. But about this not being on the exam i wud'nt say that too loud. Hope i didnt say too much....

THanks again

One of the guides I used either cisco, sybex or train signal said something about a /31... so it may be covered

mikej412

kenny504 wrote:

Hope i didnt say too much....

Up until the "when i sat the ICND" you were doing okay. It could have been a paraphrased Cisco Press Flash Card/Test Questions.... or a Sybex/Cisco Press book question.... But nooooooooooo -- you have to drag an exam into your post.

But my study break over.... so I guess you win this round....

dmaftei

DirtySouth wrote:

However, in this case apparently it is used.

Apparently?! The question starts with "using subnet zero..."

What that means is, the address that is normally reserved only for network addresses can be used as a host address.

No, it doesn't mean that. "Using subnet zero" means that first subnet is used; it has nothing to do with address assignment WITHIN the subnets.

kenny504

[/quote]No, it doesn't mean that. "Using subnet zero" means that first subnet is used; it has nothing to do with address assignment WITHIN the subnets.[/quote]

OK, so your saying all addressin the first subnet only can be usuable..and only the addresses in the first subnet??......

If u have a link or some notes to bolster what you just said can you post it also please. Because i really want this concrete. Thanks

cambei

Here is a good explanation from the horse's mouth itself:
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

kenny504

Thanks alot cambei...that helps alot.

DirtySouth

dmaftei wrote:

DirtySouth wrote:

However, in this case apparently it is used.

Apparently?! The question starts with "using subnet zero..."

What that means is, the address that is normally reserved only for network addresses can be used as a host address.

No, it doesn't mean that. "Using subnet zero" means that first subnet is used; it has nothing to do with address assignment WITHIN the subnets.

Take a deep breath, bro. Have you not ever heard the word 'apparently' used? If you're not using subnet zero then none of the addresses in the first subnet can be used as host addresses. Thats all I was trying to say.