Need help for VLSM Class B

sabregod

Hello to all


I've been practicing subnetting for hosts requirement and I can't seem to figure it out. Here's the question and my instructor answers. https://i.imgur.com/ceVki6c.png



I can't seem to know how he got those numbers. Am I doing something wrong? Here's my answer and calculation:

https://i.imgur.com/2PqKTnC.png

https://i.imgur.com/D9DuZYz.jpg



Any help will be greatly appreciated

rob42

Your first error is with R1 Fa Stub.

You have 172.128.51.0 /25. This block supports 126 Host IPs. This network will have...

First Host IP - 172.128.51.1
Last Host IP - 172.128.51.126
B/Cast IP - 172.128.51.127

...and so contravenes the 'Don't use 172.128.51.76', so you'll need to use the next block up: 172.128.51.128 /25

First Host IP - 172.128.51.129
Last Host IP - 172.128.51.254
B/Cast IP - 172.128.51.255

This leaves 172.128.51.0 -> 172.128.51.127 unused, so a part of that range can be used for the next interface: R2 Lo0 - 172.128.51.0 /26, which needs 60 Host IPs.

First Host IP - 172.128.51.1
Last Host IP - 172.128.51.62
B/Cast IP - 172.128.51.63

We can use 172.128.51.64 -> 172.128.51.75 later, but the next block needs 20 Host IPs and so will be 172.128.51.96 /27

First Host IP - 172.128.51.97
Last Host IP - 172.128.51.126
B/Cast IP - 172.128.51.127

The next block needs 10 Host IPs. We can't use the 172.128.51.64 /28 block as this has 14 Host IPs and so would again contravene the 'Don't use 172.128.51.76', so we have to use the next block of 14: 172.128.51.80 /28

First Host IP - 172.128.51.81
Last Host IP - 172.128.51.94
B/Cast IP - 172.128.51.95

We can now go back to the block that starts at 172.128.51.64 as we only need 5 Host IPs, which 172.128.51.64 /29 can provide.

First Host IP - 172.128.51.65
Last Host IP - 172.128.51.70
B/Cast IP - 172.128.51.71

The last block needs just 2 hosts, so we can use .73 and .74 on the 172.128.51.72 /30 network.

First Host IP 172.128.51.73
Last Host IP 172.128.51.74
B/Cast IP 172.128.51.75



Interface
Max #of Hosts Req
CIDR
1st H. IP
Last H. IP
B/Cast


R1 Lo1
350
172.128.48.0 /23
172.128.48.1
172.128.49.254
172.128.49.255


R2 Lo2
130
172.128.50.0 /24
172.128.50.1
172.128.50.254
172.128.50.255


R1 Fa Stub
100
172.128.51.128 /25
172.128.51.129
172.128.51.254
172.128.51.255


R2 Lo0
60
172.128.51.0 /26
172.128.51.1
172.128.51.62
172.128.51.63


Edge Lo0
20
172.128.51.96 /27
172.128.51.97
172.128.51.126
172.128.51.127


R1 Lo0
10
172.128.51.80 /28
172.128.51.81
172.128.51.94
172.128.51.95


R2 Lo1
5
172.128.51.64 /29
172.128.51.65
172.128.51.70
172.128.51.71


R2 to Edge
2
172.128.51.72 /30
172.128.51.73
172.128.51.74
172.128.51.75




I hope this is of help to you.

rob42

You're welcome b.t.w...