VLSM network design

Question

I'm studying for the CCNA exam and I want to create a VLSM scheme using RIPv2 for the following (CCNA practice example):

Network connecting 4 different locations
Each switch is fa 0/0 on the router
Every point-to-point serial connection is in the network is class C (192.168.1.0 )
Network 172.20.0.0 for each location
Use the lowest subnet number (zero is allowed)

Requirements for each location:

Dallas - 500 hosts
New York - 2000 hosts
Chicago - 1000 hosts
LA - 2000 hosts

Here's my design so far:
enter image description here

Do you see any flaws in this? How would you fix them? Or how would you design this?

Yeah, it was a practice question for the CCNA and I wanted to make sure I was starting off right. — 에이바, Apr 04 '11 at 04:25
http://www.gns3.net/phpBB/topic3382.html?sid=81633409141c15aaf5778e7858a847cd — 에이바, Apr 08 '11 at 01:29

score 6 · Answer 1 · edited Apr 13 '17 at 12:14

It looks like you're asking us to do your homework... anyway, I do see some flaws in the subnetting.

Do you really understand how subnetting works ? It doesn't seem so.

This post may help you to get the picture, but let's improve your design & skills:

Dallas : 500 hosts => 512 => /23
New York : 2000 hosts => 2048 => /21
Chicago : 1000 hosts => 1024 => /22
LA : 2000 hosts => 2048 => /21

On this part you're right. But here is the fault :

 172.20.0.1/23 = 172.20.0.0 -> 172.20.1.255
 172.20.0.1/22 = 172.20.0.0 -> 172.20.3.255
 172.20.4.1/21 = 172.20.0.0 -> 172.20.7.255
 172.20.6.1/21 = 172.20.0.0 -> 172.20.7.255

So all your networks are overlapping, which isn't good. Each address must be unique. So a correct design would be like this :

 172.20.0.0/21  = 172.20.0.0  -> 172.20.7.255
 172.20.8.0/21  = 172.20.8.0  -> 172.20.15.255
 172.20.16.0/22 = 172.20.16.0  -> 172.20.19.255
 172.20.20.0/23 = 172.20.20.0 -> 172.20.21.255

You can start from the largest networks to the smallest, with the starting address being the last+1 from the previous network.

The 172.20.16.0/22 should = 172.20.16.0 ...right? – 에이바 Apr 04 '11 at 16:49 — 에이바, Apr 04 '11 at 16:49
@에이바: indeed, copy/paste error :) – petrus Apr 04 '11 at 17:34 — petrus, Apr 04 '11 at 17:34

score 2 · Answer 2 · answered Apr 11 '11 at 16:12

2

Your Serial Connections subnet is off: You are specifying a 192.168.1.0/30 for 4 point to point connections @ 2 addresses each = 8 needed IPs. A /30 CIDR will only yield 2 available host addresses (enough for only one connection). You should have a single /30 subnet for each point to point:

192.168.1.0/30  = 192.168.1.0  -> 192.168.1.3  (Use .1 & .2)
192.168.1.4/30  = 192.168.1.4  -> 192.168.1.7  (Use .5 & .6)
192.168.1.8/30  = 192.168.1.8  -> 192.168.1.11 (Use .9 & .10)
192.168.1.12/30 = 192.168.1.12 -> 192.168.1.15 (Use .13 & .14)

answered Apr 11 '11 at 16:12

HostBits

11,776
1
24
39

Your diagram is still technically wrong though. – HostBits Apr 11 '11 at 16:19
How so? I posted a new answer the the diagram. – 에이바 Apr 11 '11 at 16:22
Yes, the new diagram is still technically incorrect. You are only specifying one of the four serial connection subnets. – HostBits Apr 11 '11 at 16:29
http://i.imgur.com/VuhGX.png – 에이바 Apr 11 '11 at 16:40

score 0 · Accepted Answer · answered Apr 11 '11 at 16:13

Switches:

New York --> LGA 172.20.0.0/21
Los Angeles --> 172.20.8.0/21
Dallas --> DFW 170.20.20.0/23
Chicago --> ORD 172.20.16.0/22

Routers:

192.168.1.13 ORD s0/1
192.168.1.14 LAX s0/0

192.168.1.1 DFW s0/0
192.168.1.2 LAX s0/1

192.168.1.5 DFW 0/1
192.168.1.6 LGA s0/0

192.168.1.9 LGA s0/1
192.168.1.10 ORD s0/0

enter image description here

VLSM network design

3 Answers3