Subnet summarisation

[Bienvenu Ngala]

Hi Everyone,

This question may look stupid but its importance is vital to me as I am learning this technolgy my question is this:

I have upgraded my network and configure 10 vlans on a L3 switch: 

                                                                               - vlan 10 ip address 192.168.10.1  255.255.255.0

                                                                               - vlan 20 ip address 192.168.20.1  255.255.255.0

                                                                               - vlan 30 ip address 192.168.30.1  255.255.255.0

                                                                               - vlan 40 ip address 192.168.40.1  255.255.255.0

                                                                               - vlan 50 ip address 192.168.50.1  255.255.255.0

                                                                               - vlan 60 ip address 192.168.60.1 /24

                                                                               - vlan 70 ip address 192.168.70.1 /24

                                                                               - vlan 80 ip address 192.168.80.1 /24

                                                                               - vlan 90 ip address 192.168.90.1 /24

                                                                               - vlan 100 ip address 192.168.100.1 /24

Now, what is the subnet summarisation of all these ip addresses or what is a unique ip address summarising all these subnets.

Best regards,

BEN

[Ashis Patra]

We can summarise that to 192.168.0.0/23.

[Bienvenu Ngala]

Hi A. Patra,

Many thanks for your response. With /23 the subnet mask will be 255.255.254.0  I beleive this is right.

Please could you explain to me briefly how you arrived to this, so that next time I can help myself and others. Please help.

Best regards,

Ben

[Ashis Patra · Accepted solution]

I made it wrong....

Correct one is....

We can summarise that to 192.168.0.0/17 OR as 192.168.0.0/16

See, when you are saying about summarisation then, one thing you must know that its good to summarise two adjacent ip address...

And according to ur data all ip's are not adjacent ip's..

We can summarise these ip's to 192.168.0.0/16 or /17, but the problem is the L-3 switch will have all networks

that you configured and also network like 192.168.101.1,

192.168.11.1- 19.254

192.168.21.1- 29.254

192.168.31.1- 39.254,

192.168.41.1- 49.254,

192.168.51.1- 59.254,

192.168.61.1- 69.254,

192.168.71.1- 79.254,

192.168.81.1- 89.254,

192.168.91.1- 99.254,

192.168.101.1 and so many...

So, if a packet having destination  192.168.11.1, then the packet will think that this L-3 switch has its destination.

So, it will go to that switch but the switch will drop the packet as it has no such entries like 192.168.11.1.

So it will increase ur Switch's CPU cycles and can decrease Switch's performance....

Another thing you always should know that

Summarisation means combining smaller networks to make a single large networks.

Subnetting means dividing larger networks to small networks..