Inter-vlan issue 'IP' Overlaps with gigabitethernet0/0/0

[Anonitachi]

---

Hey all,

A bit of hand-holding needed.

Setting up two VLANs on a router - 10 & 20, and I'm completely stumped and could use advice. The first thing that I've setup is the gigethernet0/0/0 with an IP and, 0/0/1 with no IP (fake ip address used here)

Router1

Interface Gi0/0/0

ip add 80.0.0.1 255.0.0.0

No shut

..

Interface Gi0/0/1

No ip add

No shut.

When attempting to setup VLAN 10, I use the following and getting the overlap error.

Interface G0/0/1.10

Encapsulation dot1q 10

ip add 80.0.10.1 255.0.0.0

80.0.0.0 overlaps with GigaEthernet0/0/0

I'm a noob, and can't get my head around it.

I'm not even sure of the exact meaning of the 'overlap' issue and why it's doing what it's doing.

All help appreciated

Many thanks

Anon

[neilneil]

Hi Rick,

As a basic configuration the level 3 router had 2 interfaces,

The first one being Gi0/0/0 which has an IP address of 99.0.0.1 255.0.0.0

The second one was Gi0/0/1 with no IP address.

The selection method is just based on a customer number and my one means that 16 further subnets are to be created.

Firstly, which interface would i put my sub interfaces on for the 2 VLANs. I am assuming it would be on the Gi0/0/1 interface using the 172.16.0.0/24 network like you say, but how do i then break it down for the subnetting?

would VLAN 10 be 172.16.10.1/29 and VLAN 20 be 172.16.20.1/29, I'm so sorry to be a pain. I hope the attachments explains it further, Cisco1 is the network and cisco2 was the basic configuration i did.

Thankyou.

Neil

[Richard Burts]

Neil

I believe that we are getting closer. First let me respond to this "would VLAN 10 be 172.16.10.1/29 and VLAN 20 be 172.16.20.1/29" No they would not. The instructions were to subnet 172.16.0.0/24. That means that for every subnet the first 3 octets will always be 172.16.0. So you would not have 172.16.10 or 172.16.20.

And yes it would make sense to have the vlan subinterfaces on G0/0/1. Note that interface G0/0/1 would still not have any IP address. The IP addresses would be configured on the subinterfaces for each vlan.

Then let me suggest a solution: At this point I will take the approach of keeping it simple and create 16 subnets each of which has 16 addresses. If you check with the tutor and find that the intent is that subnet size should be variable then I will change my suggestion. So 16 subnets each with 16 addresses would look like this

172.16.0.0 is the first subnet. The instructions say it is for vlan 10. So perhaps the router subinterface would be 172.16.0.1

175.16.0.16 is the second subnet. The instructions say it is for vlan 20. So perhaps the router subinterface would be 172.16.0.17

172.16.0.32 is the third subnet. I am not clear where or how this would be used.

172.16.0.48 is the fourth subnet. I am not clear where or how this would be used.

172.16.0.64 is the fifth subnet. I am not clear where or how this would be used.

172.16.0.80 is the sixth subnet. I am not clear where or how this would be used.

172.16.0.96 is the seventh subnet. I am not clear where or how this would be used.

172.16.0.112 is the eighth subnet. I am not clear where or how this would be used.

172.16.0.128 is the ninth subnet. I am not clear where or how this would be used.

172.16.0.144 is the tenth subnet. I am not clear where or how this would be used.

172.16.0.160 is the eleventh subnet. I am not clear where or how this would be used.

172.16.0.176 is the twelfth subnet. I am not clear where or how this would be used.

172.16.0.192 is the thirteenth subnet. I am not clear where or how this would be used.

172.16.0.208 is the fourteenth subnet. I am not clear where or how this would be used.

172.16.0.224 is the fifteenth subnet. I am not clear where or how this would be used.

172.16.0.240 is the sixteenth subnet. I am not clear where or how this would be used.

[neilneil]

Hi Rick,

That was helpful, thank you very much. Spoke to my tutor he was not helpful really as they were not allowed to help in any way, just explained about borrowing host bits to make the subnet which i understand (I think). My understanding is the 172.16.0.0 would have the subnet mask 255.255.240.0 for 16 subnets then??

[Richard Burts]

Neil

Perhaps my reaction is more about semantics than anything else. The idea that a tutor is not allowed to help you understand a concept is very strange. I would think that it was more like a proctor. But that distinction is not relevant to this discussion.

You are close but not quite right about the mask. For these subnets the mask would be 255.255.255.240.

[neilneil]

Hi Richard,

Yes, i agree, the tutor is behaving more like a proctor but i have spent the morning watching videos.

My take now is the following commands are to be used:

enable

config t

interface Gi0/0/1.10

encapsulation dot1q 10

ip address 172.16.0.0 255.255.255.240

no shutdown

interface Gi0/0/1.20

encapsulation dot1q 20

ip address 172.16.0.15 255.255.255.240

no shutdown 

exit

wr

Do the ip address look correct to you now Rick?

Many thanks,

Neil

[Richard Burts]

Neil

Your approach is right but your IP addresses are not right.

172.16.0.0 is a reserved IP address for your subnet and can not be used as a host address. This address is the base address of the first subnet.

172.16.0.15 is a reserved IP address for your subnet and can not be used as a host address. This address is the broadcast address of the first subnet.

For the first subnet (for vlan 10) the available addresses start at 1 and go through 14. Many people would use .1 as the router subinterface address (sort of logical since the router is the first device in the subnet). I might use .14 for the router subinterface address. I have seen situations where the router uses the lowest address of the subnet and then someone configures a PC using that address and connects it to the network. You can use any address you want out of those available addresses.

For the second subnet (for vlan 20) the available addresses start at 17 and go through 31. Same logic applies about which address to use for the router.

[neilneil]

Hi Rick,

Got it, thanks for your help mate. Greatly appreciated.

Neil

[Richard Burts]

Neil

You are welcome. I remember in my early days of networking the concepts used in subnetting seemed challenging: subnet masks, how big can a subnet be, how big is a particular subnet, what address ranges go into which subnet, etc. With practice and with some experience it does get easier. I think you will experience this and some day you will be able to offer suggestions to someone less experienced than you about subnetting.