subnet -2 confused

lon

When you subnet, do you -2 from the subnet id(network id) and the host id or just the host id? Books I've read shows -2 from the host id only and some will do both. Which one applies to the ccna test?

Miek

On the exam there will be a reference if subnet zero is used or not.
When subnet zero is used, you can use the all 0 and all 1 subnets (no -2).

When subnet zero isn't enables, you cannot use these 2 subnets and therefor you have to do -2.

For the host portion you will always need to -2. One for the network address and one for the broadcast.

Hope this helps.

Greetz

Miek

Drakonblayde

According to Lammle's book, subnet 0 and all-ones are not valid on all current Cisco exams. Whether or not that's changed, I dunno, but yeah, subnet 0 and all-ones have always been usable.

I felt cheated when I found this out after having been taught otherwise

mwgood

You might find this link interesting with regard to this subject...


http://www.cisco.com/warp/public/105/40.html

-Mike

lon

Thanks for the link.

This is one area Im having a hard time with, esp. with finding a range of a giving IP and its network id and broadcast value. I've read many tutorials on the net, some shows -2 from the net id and host id and some don't. This totally messed me up. Once its cleared, I'll take my CCNA.

I have taken some sample CCNA tests, if it were not for the multiple choices, I'd be screwed in this area.

Thanks for the advice.

mwgood

Well, if you are having trouble with subnetting - take a look at this - http://www.techexams.net/forums/viewtopic.php?t=4291

I tried to simplify the whole topic in my post there - and there are some good links in that thread to excellent websites.

Once you get it, it's a breeze )

-Mike

Drakonblayde

Ok, to break it down...

When subnetting, tradition teaches that you do not use the first subnet because it contains the unsubnetted network ip (which I call the major network ip) and you don't use the last subnet because it contains the major network broadcast address.

Reality states that you can use those subnets just fine.

However, for the Cisco exams, apparently, tradition holds sway. I'm taking Todd Lammle's word though. So apparently for the exam, we do the Cisco way and not use first or last, but in the real world, we can!

No, for your subnets, you always lose two IPs. Always.

Let me say that again -

Always.

The first IP is the network ID, and therefore no host can be assigned that id. If it helps, think of it along the lines of the router reserving that address for it's own use (it may not actually assign that address to an interface, it just uses the network ID to make forwarding decisions. Obviously, it would get a little confused if that address was assigned on a local host).

The last ip in the subnet is the broadcast IP. Think of it as being reserved for all hosts.

So basically, while there is some ambiguiety as to whether you drop 2 from your overall # of networks, you always drop 2 from your total pool of actual host addresses

rossonieri#1

first : determine what you are up to -> go for subnets or hosts, second the octet always contains either 0 subnet with 8 bit host or 0 host with 8 bit subnet... ex. : if you are using 2 bit subnet so there will be 8 - 2 = 6 host bits and vice versa. configuring that dont forget your major ip class.. and so on... i am not speaking any vlsm or cidr here, because those are another things to configure...

smooth7905

ccna questions will specify
if ip subnet 0 is enable or not

smooth7905

the first address in a given subnet is the subnet id and the last in a given subnet is the broadcast address for that subnet