Another way to look at it is that you have one long contiguous line of addresses that you need to break up into different size groups that must also be contiguous.
For example: 192.168.1.0 /24 Assume we need three networks (nets a&b) with 40 hosts, two networks (nets c&d) with 25 hosts, and 3 networks (nets e,f,&g) with 12 hosts. Our available subnet area is 192.168.1.0 - 192.168.1.255 Shown graphically (hope this doesn't get munged in transmission; if so copy and paste into word using fixed width and 10). .0 .255 /24: |-------------------------------------------------------------------------------| .128 /25: |-------------------------------------|-----------------------------------------| .64 .192 /26: |-----------------|-------------------|-------------------|---------------------| (a) ( b) .160 .224 /27: |-----------------|-------------------|---------|---------|----------|----------| (c) (d) .208 .240 /28: |-----------------|-------------------|---------|---------|-----|----|----|-----| (e) (f) (g) >From this we can see that Subnet blocks 192.168.1.0 & .64 are used for nets a & b. Blocks .128 & .160 are used for nets c & d, while blocks .192, 208 & 224 are used for nets e, f, & g. This leaves one block, .240 for future use or to further subnet for /30's to address serial links. By looking at it visually, there isn't any danger of overlapping previously assigned blocks. It also shows us where supernetting will occur so we can properly assign the blocks for easy aggregation. Since the blocks must be recombined for supernetting in the same manner they were subnetted, we can see that nets a & b could be put on one router and we would only advertise a /25 (192.168.1.0) upstream. Likewise, blocks .128 & .160 could be on a second router and advertised as a /26 (192.168.1.128). We can also see that we couldn't put nets a-d on the same router and advertise as a /25 because they don't all come from the same /25 block. We would have to use two advertisements, a /25 & /26, if they were on the same router. The same method can be used for address ranges that cross octet boundries: 172.16.0.0 /16 0.0 255.255 /16: |---------------------------------------------------| 128.0 /17: |-----------------------|---------------------------| and so on..... I find showing it visually to my students makes understanding a lot easier. HTH, Prof. Tom Lisa, CCAI Community College of Southern Nevada Cisco ATC/Regional Networking Academy "Cunctando restituit rem" John Neiberger wrote: 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 **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=75127&t=75127 -------------------------------------------------- **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
