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Subseting question

Posted on 2008-10-23
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Last Modified: 2012-05-05
I  just starting to grasp subnetting...and I need you inputs.
I am in process of setting up a main office network, plus two branch offices with VPN.
The entire network has 12 user groups with 10 PCs, and it will be set up a subnet 172.10.10.1 with a mask of 255.255240.0.
Each subset is connected to a layer 2 switch, and all layer 2 switches are connected to a  core cisco layer 3 switch.
A file server and a web server is connected to cisco layer 3 switch.
Couple questions to clearly my thought:

Net Addr.
172.10.10.0    .1~.14        (B.A).15
172.10.10.16    .17~.30        (B.A).31
172.10.10.32    .33~.46    (B.A).47
172.10.10.48    .49~.62    (B.A).63
172.10.10.64    .65~.8    (B.A).79
172.10.10.80    .81~.94    (B.A).95
172.10.10.96    .97~.110    (B.A).111
172.10.10.112    .113~.26    (B.A).127
172.10.10.128    .129~.142    (B.A).143
172.10.10.144    .145~.158    (B.A).159
172.10.10.160    .161~.174    (B.A).175
172.10.10.176    .177~.90    (B.A).191
172.10.10.192    .193~.206    (B.A).207
172.10.10.208    .209~.22    (B.A).223
172.10.10.224    .225~.238    (B.A).239
172.10.10.240    .241~.254    (B.A).255

1. Let's say Group #1 (172.10.10.17-30) will be adding 10 more PCs in the future,  but this subset can only has 14 hosts.
    can I add the unused subset (72.10.10..225-238) to this group #1 in the future by connecting both subset to the same layer 2 switch?

2. Is this correct to use Broadcast Address as switch's IP address?

3. Will the network run into conflicts if I place two branch offices into 172.10.10.176  and 172.10.10.192 subsets?
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Question by:compmlb
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Expert Comment

by:z_kwecka
ID: 22790796
1. Yes. It is not the greatest solution but it will work. Hosts in the different subnets will not be able to communicate directly (they will need to communicate through the layer 3 switch), but nowadays, most traffic will be heading towards the centralised resources and the Internet, so this is not a problem. You will need to set two IP address (two gateways) for the layer 3 port on the Layer 3 switch connected to this layer 2 switch.

2. No. Broadcast adddress is used for the initial communication of devices. Instead, you can use private IP addresses for administrative purposes, this way you won't have to waste the address space.

3. Don't think so. What kind of conflicts? Where do you see the problem in this setup? On first (very short) sight I can't see anything wrong with this.

good luck
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by:compmlb
ID: 22791069
Thanks, z_kwecka.  As for question #1, this is my plan:

Current group #1: 172.10.10.17-30
Layer 2 switch IP: 172.10.10.17

Future group #1:   72.10.10..225-238
Layer 2 switch IP: 172.10.10.225

Is it correct to use 172.10.10.17 and 172.10.10.225 IPs for the layer 3 port on the Layer 3 switch connected to this layer 2 switch?

BTW, What will be  the greatest solution for question #1?
I have increased point value to 500, Thanks again.
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Accepted Solution

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z_kwecka earned 500 total points
ID: 22794598
172.10.10.17 and 172.10.10.225 set as gateways will work fine. It could add tiny routing problems in any other scenario (route agregation), however, since you are using only one layer 3 device in your headquarters (or just a few) this won't be a problem at all.

As to the solution I would choose:
Your layer 3 switch sould be able to support varible length subnet mask, thus, I would make 4 different subnets of different size. One for each branch, one for the server, and one for all the hosts in the headquarters. From your numbers it looks that you do not have more than 100-120 hosts in the headquartes. They should be able to work together on a switched network. Switched networks are faster than routed, also you may then add aditional host wherever you want without doing any changes to the subnets.

(In this scenario you may want to observe the broadcast levels. If you are using some applications generating a lot of broadcasts, and the broadcast levels are too high you may want to split the large subnet a little bit more (split in two at first).
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