when should i turn off ip subnet-zero ?

If you aren't running Sun 3 systems, you probably don't need to leave subnet 0 reserved.

There used to be a reservation of the first subnet within a network, for use in broadcasting, because at the time either the first or last address in a range could be used as the broadcast address. The use of the first address as the broadcast address went away a long time ago, but Cisco equipment still reserves it in case you are running old equipment.

Hi There,

When you subnet a major network, you will get multiple subnets. e.g. If you subnet 192.168.1.0/24 into /26 subnets, then you will get following four subnets:

192.168.1.0/26 192.168.1.64/26 192.168.1.128/26 192.168.1.192/26

If you turn off ip subnet-zero, then you will NOT be able to use subnet zero (in this case 192.168.1.0/26). I have always used ip subnet-zero command, and it has never created a problem for me. So I would never recommend anyone to turn it off.

Hope this helps.

Best Regards Nandan

Correct [in the context I snipped]

Okay, fine, a valid observation of your personal experience.

But that statement tends to indicate that you do not understand why ip subnet-zero was invented. If you knew -why- subnet zero was not normally usable, then you would not say that you would "never" recommend that anyone would turn it off, because some day you may encounter a network that relies upon subnet zero being reserved.

It was invented because of issues that existed over a decade ago, and are now obsolete. Unless you go into a shop that is still running

10-year-old systems, you can safely forget about it. Unfortunately, Cisco is really slow at changing their default configuration settings.

In other words, knowledge of this is about as useful as knowing how to administer Windows 3.x systems.

[ip subnet-zero]

Yup. Now turn the logic around: If you were to happen to go to a shop running old systems (or which couldn't afford to change after having run those systems for a long time), then you would need to know the command, and in those circumstances you would recommend its use.

The poster I was replying to was saying that he would *never* recommend use of the command. He might never encounter those legacy systems (outside of a cisco cert exam ;-) ) but if he -does- happen to encounter them, how is he going to deal with the situation if he has denied himself use of the appropriate tool?

And of course on the Internet, every word must be taken absolutely literally.

On a technical newsgroup, when giving technical advice to others, "I would never" conveys a very different message than "I have never".

I was referring to interpreting "never" as a literal absolute. Like if someone were to say "I would never kill someone", although in fact there are most likely some extreme situations where they actually would. Or, if you want a technical example, one might say "Never just cut power to a computer, you should always perform a graceful shutdown"; the qualification that this only applies when a graceful shutdown is possible is obvious.