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TOPIC: Differences between VLSM and CIDR

Differences between VLSM and CIDR 7 years 4 months ago #30273

  • NeoTech
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hey guys i may need to go back over my notes for this but can anyone clearly define the difference between CIDR and VLSM for me?

Is CIDR simply the notation of a VLSM? that whole part of CCNA has packed its bags and left my brain for some reason!
:oops:

Neo
Tech
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Re: Differences between VLSM and CIDR 7 years 4 months ago #30274

CIDR = Prefix length.

VLSM is taking a subnetted address and re-subnetting one of the subnetted networks.

IE, if you have 192.168.10.0/27, you have networks:
192.168.10.0
192.168.10.32
192.168.10.64
192.168.10.96
etc.

You can take (for example) 192.168.10.32/27, and make it 192.168.10.32/29, thus Varying the Length of the Subnet Mask. You can now have networks:
192.168.10.32
192.168.10.40
192.168.10.48
192.168.10.56


If we assume 192.168.10.64 is in use, as are all the addresses that follow in the original /27 scheme, then you have to stop there. If the .64 address scheme is not being used you can keep going with the smaller subnet size until you reach an address that is in use. Obviously you can't duplicate addresses...
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Re: Differences between VLSM and CIDR 7 years 4 months ago #30400

  • pedenski
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wow thanks.. ^^
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Re: Differences between VLSM and CIDR 7 years 4 months ago #30414

y stopping at .64??? /29 can take us upto .248 , rite...? plz correct me..am lil bit confused

CIDR = Prefix length.

VLSM is taking a subnetted address and re-subnetting one of the subnetted networks.

IE, if you have 192.168.10.0/27, you have networks:
192.168.10.0
192.168.10.32
192.168.10.64
192.168.10.96
etc.

You can take (for example) 192.168.10.32/27, and make it 192.168.10.32/29, thus Varying the Length of the Subnet Mask. You can now have networks:
192.168.10.32
192.168.10.40
192.168.10.48
192.168.10.56


If we assume 192.168.10.64 is in use, as are all the addresses that follow in the original /27 scheme, then you have to stop there. If the .64 address scheme is not being used you can keep going with the smaller subnet size until you reach an address that is in use. Obviously you can't duplicate addresses...
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Re: Differences between VLSM and CIDR 7 years 4 months ago #30428

You cannot duplicate addresses that may already be in use. For example:

If you are given a network address from your ISP, lets say 190.168.10.32/27. The ISP says that you can ONLY use this network address range. All other numbers are in use. You can stick with a flat network of 30 hosts, OR re-subnet the number into smaller networks with fewer hosts, which is VLSM. You can change the mask to /28, /29, or /30 if you want, although /30 are usually saved for P2P router connections.
So, lets take 190.168.10.32/27 and make it 190.168.10.32/29. That gives us four networks of 6 hosts per network rather than the flat 30 host network foisted on us by the ISP:

190.168.10.32 First host .33 last host .38, broadcast .39
190.168.10.40 First host .41 last host .46, broadcast .47
190.168.10.48 First host .49 last host .54 broadcast .55
190.168.10.56 First host .57 last host .62 broadcast .63

Whilst you are correct that the /29 does indeed stop at 248, we have to stop where we have because the ISP has told us that all the other possible network addresses and combinations are in use by others. So we have taken our original flat 30 host network and resubnetted it into 4 networks with 6 hosts per network, and we haven't nicked any numbers that are in use by others as per the ISP's conditions.
All we need now are some VLAN's and some DOT1Q trunking and maybe some subinterfaces, and we're off to the small business network design awards with a strong possibility of winning something!
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Re: Differences between VLSM and CIDR 7 years 3 months ago #30647

  • NeoTech
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Awesome, so basically VLSM is using more than one subnet in a single network (ie. creating a subnet within a subnetted network...lol)...

...and CIDR is simply the /* bit notation used in classless networking (which a layer 3 device can understand)?

Neo
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