Help Needed

151.242.16.49 would be class B in a classful environment with a network mask of 16 bits. An additional 7 bit subnet mask puts the total mask at 23 (i.e.: 255.255.254).

Subnet ID would be: 151.242.16.0 Host address range: 151.242.16.1 through 151.242.17.254 Subnet Broadcast address would be: 151.242.17.255

Best Regards, News Reader

Thought you might benefit from understanding how to arrive at the answer rather than giving you the answer (as in my other post).

Hopefully you can recognize that the address would have a natural mask of 16 bits in a classful environment (class boundaries are easy to remember since there are few of them), and that an additional 7 bit subnet mask results in a total mask of 23 bits. If you understand binary, you should be able to determine that a 23 bit mask can be represented as 255.255.254.0, which will be helpful.

The third octet of the mask is the "octet of interest" given that it is the octet where the boundary exist between the network portion and the host portion.

If you subtract the value of the mask's "octet of interest" (254) from

256 you will get the "network increment" (2 in this case). Yield to the "256", commit it to memory, don't rack your brain trying to understand it, it works. ;>)

The "network increment" is the increment between subsequent subnets IDs (occurs in the 3rd octet in this case).

The subnet IDs would be as follows:

151.242.0.0 a legitimate subnet if your equipment is configured to support "subnet 0". 151.242.2.0 151.242.4.0 151.242.6.0 etc. . note how the subnet IDs increment by 2. 151.242.14.0 151.242.16.0 your subnet 151.242.18.0 next highest subnet.

Your subnet is 151.242.16.0 as determine by the combination of your host IP 151.242.16.49 and the 23 bit mask. The first available host address (151.242.16.1) is 1 greater than the subnet ID (151.242.16.0). The subnet broadcast address (151.242.17.255) is 1 less than the "next" subnet ID (151.242.18.0). The last available host address (151.242.17.254) is 1 less than the broadcast address (151.242.17.255).

There are many different thought processes used by admins, each partial to their own way. I find it beneficial to subtract the decimal value of the mask's "octet of interest" from 256 to determine the subnet increment. The rest becomes clear after that.

Best Regards, News Reader

People (and router manufacturers) really really really ought to stop assuming anything about netmasks from the prefix value. It's only been

10 years now...

Sam

Please don't take this as a dig at anyone, it's not.

I agree.

I read the original post as a netmask of 7 bits.

254.0.0.0

I try to avoid using the "sub" term at all - ever.

It's just a network part and a host part. Well, except sometimes:-)

I do use "subnet" when talking to lay users since they expect it but in professional circles it mostly adds opportunity for confusion. (Luckily,) lay users are confused about networks anyway.

The original poster did not say "netmask", and he did not say "supernet" or "CIDR".

If he had, my answer would have been different.

He specified "subnet mask". The words exists for a reason, and convey a real world scenario in many environments.

If you are going to make assumptions, why not respect the person enough to assume they know what they are asking for?

Exactly. There is often a network part, a subnetwork part, and a host part.

Not if you use the words correctly.

Best Regards, News Reader

I respect your response and your professionality(?) and it's neither you nor the OP I'm getting at, its the manufacturers and trainers (this looks a little like homework) who persist in thinking classfully. We should be long gone from there.

But no one should be thinking that way any more - they should be thinking masks, prefixes and aggregates.

I use it too, but I shouldn't. :-)

Then we have a mission to educate and inform (and possibly entertain along the way)!

Sam

professionalism ;>)

Generally, I agree.

However, when "default routing" is impacted by the presence or absence of a command like "ip classless", having some awareness of clasful boundaries still has some relevance.

Best Regards, News Reader

I *knew* there was a word I ought to know. I've only been a native English speaker for about 50 years...

Sam