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Different VLSM and IP Planning

What are the rules for vlsm assignment for ip planning..

For example;

In my office I assigned that network address to one subnet of Router A:

192.168.3.11100000/27

Say; one of the host's ip address is : HOST1: 192.168.3.11100010 /27

But in the same office I assigned different vlsm to another subnet of different router, say, Router B:

192.168.3.11100010 /29

If one of the host of this network wants to communicate with HOST1(192.168.3.11100010) It can't happen because after mask anding, it thinks it is in the same lan.

so there is a collision here.

Right?

So what are the rules for ip planning with different vlsm? How can we avoid such collisons?

3 REPLIES
New Member

Re: Different VLSM and IP Planning

Don't really understand your ip addresses but I gress the last 8 digits there is a binary representation of the final octet of the ip address.

Yes, you have a conflict, the /27 overlaps the /29 and whoever is on or connects with the /27 net will beleive the /29 net you are using is local thus no routing possible.

The /27 represents the netmask 255.255.255.224 and is a block of 32 addresses.

The /29 represents netmask 255.255.255.248 and is a block of 8 addresses.

Sent from Cisco Technical Support iPad App

Silver

Different VLSM and IP Planning

sawquecisco wrote:

What are the rules for vlsm assignment for ip planning..

For example;

In my office I assigned that network address to one subnet of Router A:

192.168.3.11100000/27

Say; one of the host's ip address is : HOST1: 192.168.3.11100010 /27

But in the same office I assigned different vlsm to another subnet of different router, say, Router B:

192.168.3.11100010 /29

If one of the host of this network wants to communicate with HOST1(192.168.3.11100010) It can't happen because after mask anding, it thinks it is in the same lan.

so there is a collision here.

Right?

So what are the rules for ip planning with different vlsm? How can we avoid such collisons?

Find someone who actually knows IP and does the design for you.

Or download something like the Solar Winds advanced IP subnet calculator and check if your subnets overlap before putting them into your design.

Cheers.

New Member

Re: Different VLSM and IP Planning

Hi sawquecisco,

Keep in mind this:

  1. The addresses of machines on a given network must be contiguous.
  2. On two given network, the addresses does NOT overlaps.
  3. Try to have networks with address ranges that permit you to have masks which will help for routing.

(for the following examples, we suppose that the network root is 192.168.0)

  • Example for 1:

You can have on network A machines with IP@ .1,.2,.3,.4,.5,.6,.30, and on network B machines with IP@ .33,.34,.35,.36,.37,.38,.60. But it will be an error if you use addresses .16,.17 for network B. Idem if you use .40, .50 or .59 in network A.

  • Example for 2:

subnetwork A: @IP : .1 -----------------------------. 31

subnetwork B: @IP : .16-----------------------------.63

not good!

You can not use (sub)network addresses .1 to .31 on a network A and use addresses .16 to .63 for network B. In this

case, IP@ .16 to .31 are on the TWO networks!

  • Example for 3:

You can use subnetwork .0/27 on network A, .32/27 on network B. A router that federate this network can use .0/26 to manage routes for this two networks. (in /27, the IP@ by networks are .1 to .31 ; .32 to .63 ; .64 to .95 ; etc, while in /26 they are .0 to .63; .64 to .127; etc).

Complement:

Suppose that you have the following 4 others networks with /28 :

IP@ for network C: .64 to .79;

IP@ for network D: .80 to .95;

IP@ for network E: .96 to .111;

IP@ for network F: .112 to .127

the route 192.168.0.0/25 will federate network A to network F

Hope that will help you.

sadavir-ap-sampath.

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