The key is that you must completely unlearn classful thinking. Forget that you ever learned it. Completely ignore any prior classful subnet boundaries that you were forced to memorize. It's all just one big IP address space that you choose to carve up any way you like. As long as you do it correctly and don't have any overlap the subnetting scheme is up to you.
Another helpful tip: don't ever use classful terminology any more! Don't say "Class A" to refer to an 8-bit prefix or subnet mask; don't say "Class C" to refer to a 24-bit mask, or /24. That will help move your brain away from that type of thinking. Think of your address space as a big pie, and each time you cut a segment in half you're adding one more bit to the subnet mask. Here's an example: You start with 10.20.30.0/24 (255.255.255.0) and we'll think of that as a whole pie. You don't need that many addresses in your subnet so you decide to break it up into smaller pieces. What do you do? Cut your pie in half (draw this out, it helps!). Your pie now has two halves and these represent two subnets with /25 masks with no overlap. Let's say you want to further subnet one of those subnets. Cut it in half again! You now have a /25 and two /26s with no overlap. If you further cut one of those /26 subnets into two pieces you have two /27s. See how easy that is? Draw this out on paper and write down your subnet information as you go, like this: 10.20.30.0/24 (10.20.30.0-255) becomes 10.20.30.0/25 (10.20.30.0-127) and 10.20.30.128/25 (10.20.30.128-255) 10.20.30.128/25 further subnetted becomes 10.20.30.128/26 (10.20.30.128-191) and 10.20.30.192/26 (10.20.30.192-255) And so on... practice it this way for a while and after a short time it will be second nature for you to subnet existing networks without accidentally overlapping them. HTH, John >>> Steven Aiello 9/9/03 12:03:06 PM >>> I was stuck on the idea that you could ONLY re subnet a remaining piece of a subnetwork. And not apply a mask to the whole span of the total available network. You can (unless I'm incorrect here) you just have to watch out for address over lap neer your subnetwork boundries. I think I got it. Man I love this news group! Steve Priscilla Oppenheimer wrote: > Reimer, Fred wrote: > >>No offense, but this is CCNA material. > > > Do they still teach classful for CCNA, though? Perhaps the only thing that's > hard for him is that 192.168.24.0 has a mask of 255.255.255.0 in a classful > system. Moving the prefix over to the left of that classful boundary isn't > something they teach for CCNA yet. (They will soon. The new Networking > Academy books teach it from the start now.) > > Priscilla > > >>If you are going for >>your CCNP, then >>you should already have your CCNA and know the answer. But >>anyway... >> >>If you need a network with 400 hosts, the smallest subnet would >>have a /23 >>mask. So take the first part of your given network and assign >>it to that: >> >>192.168.24.0/23 (192.168.24.0-192.168.25.255) >> >>Then you need one with 200 hosts. Well, that could fit within >>a /24 subnet, >>so assign the next available to that: >> >>192.168.26.0/24 (192.168.26.0-192.168.26.255) >> >>Now you only have 192.168.27.0/24 left from the original >>192.168.24.0/23 >>(which covered 192.168.24.0-192.168.27.255). You need two >>50's, so that >>should fit within /26 subnets each. Assign them: >> >>192.168.27.0/26 (192.168.27.0-192.168.27.63) >>192.168.27.64/26 (192.168.27.64-192.168.27.191) >> >>Finally, you need three subnets that can have two hosts each, >>which would >>fit within /30 subnets. So assign: >> >>192.168.27.192/30 >>192.168.27.196/30 >>192.168.27.200/30 >> >> >>Fred Reimer - CCNA >> >> >>Eclipsys Corporation, 200 Ashford Center North, Atlanta, GA >>30338 >>Phone: 404-847-5177 Cell: 770-490-3071 Pager: 888-260-2050 >> >> >>NOTICE; This email contains confidential or proprietary >>information which >>may be legally privileged. It is intended only for the named >>recipient(s). >>If an addressing or transmission error has misdirected the >>email, please >>notify the author by replying to this message. If you are not >>the named >>recipient, you are not authorized to use, disclose, distribute, >>copy, print >>or rely on this email, and should immediately delete it from >>your computer. >> >> >>-----Original Message----- >>From: Steven Aiello [mailto:[EMAIL PROTECTED] >>Sent: Tuesday, September 09, 2003 8:02 AM >>To: [EMAIL PROTECTED] >>Subject: Please Help - CIDR - How the bits work [7:75050] >> >>I just started my routing class for my CCNP. We are covering >>CIDR. The >>book is VEEEEEERY vague on how the bit patterns break down and >>are used. >> >> >>This was a problem posed in one of my CCNP labs >> >>I have network number >> >>192.168.24.0 / 22 >> >>from this I need >>networks with >> >>400 hosts >>200 hosts >>50 hosts >>50 hosts >>2 hosts (for serial int - no ip un-numbered allowed ) >>2 hosts >>2 hosts >> >>Also no NATing >> >>Thanks all I really could use the help >> >>Steve >>**Please support GroupStudy by purchasing from the GroupStudy >>Store: >>http://shop.groupstudy.com >>FAQ, list archives, and subscription info: >>http://www.groupstudy.com/list/cisco.html > **Please support GroupStudy by purchasing from the GroupStudy Store: > http://shop.groupstudy.com > FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html **Please support GroupStudy by purchasing from the GroupStudy Store: http://shop.groupstudy.com FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=75094&t=75094 -------------------------------------------------- **Please support GroupStudy by purchasing from the GroupStudy Store: http://shop.groupstudy.com FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
