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

[Martin L]

yes, true, it is because of default class A mask is applied, that is /8 or 255.0.0.0. This means that network 80.0.0.1 255.0.0.0 belongs to range from 80.0.0.1 to 80.255.255.254.  Now, you can see that IP 80.0.10.1 falls into that range, hence, you cannot use it.  You can change mask , for example, from 255.0.0.0 to 255.255.255.0 to avoid issues. I guess you have not Study subnetting yet.

meaning of the 'overlap' is that L3 device like router and L3 switch must have all its interfaces in different networks or subnets.  Router roles is to connect and forward packets between different networks.   From above you can see that IP 80.0.10.1 falls into 80.0.0.1 to 80.255.255.254 range.  Those 2 IPs belong to the same network /8 and not different networks. 

Regards, ML
**Please Rate All Helpful Responses **

[Anonitachi]

Thanks for the prompt response, Martin.

I did try 255.255.255.128 and 255.255.0.0 (for 64 hosts) but that didn't take either.

G0/0/0, G0/0/1 & G0/0/1.10 are all up.

G0/0/0 has an ip and mask of 80.0.0.1/8 (as you said) but, still getting the overlap issue.

[Richard Burts · Accepted solution]

It seems that you did not really understand the explanation from Martin. If you configure an interface with 80.0.0.1/8 then any other interface or subinterface that uses 80 in the first octet will overlap with that network. 

[Martin L · Accepted solution]

that's because 255.255.0.0 includes 255.255.255.128 depending on first 2 octets which gives us network id,  

try this ( copy and paste):

conf t

Interface Gi0/0/0

ip add 80.0.0.1 255.255.255.0

!

Interface G0/0/1.10

Encapsulation dot1q 10

ip add 80.0.10.1 255.255.255.0

!

Interface G0/0/1.50

Encapsulation dot1q 50

ip add 80.0.50.1 255.255.255.0

!

you can use show ip route to see what network id for interface x. 

Regards, ML
**Please Rate All Helpful Responses **

[Anonitachi]

It has worked @martin. Many thanks!

So was my issue the 255.0.0.0? Subnetting does blow my mind a bit.

Two further steps that I don't understand..

1) All of this was to get the ball rolling into VLSM and now that is talking about 172.*** (?) with VLAN 10 = Subnet 0 and VLAN 20 = Subnet 1

2) And could you explain "..associate interfaces 6 thru 12 with VLAN 10 and associate interfaces 13 thru 18 with VLAN 20" and how I would go about that? Would I need to configure each interface individually or is 6 thru 12 auto-applied depending on an input/range?

Thanks again

[Richard Burts · Accepted solution]

You have asked several questions. Here are my responses:

- "So was my issue the 255.0.0.0?" Yes the mask of 255.0.0.0 was the thing that created the overlap.

- "1) All of this was to get the ball rolling into VLSM and now that is talking about 172.*** (?) with VLAN 10 = Subnet 0 and VLAN 20 = Subnet 1" I am not sure what the question is here. So let me explain several things and hopefully it may address your question. Some IP addresses are identified as Public addresses in the Internet. These Public addresses are registered to organizations and are unique (no one but the registered owner should use them). There are also IP addresses that are identified as Private addresses. Anyone can use those addresses. Networks 172.16.0.0 through 172.31.255.255 are some of the Private IP networks. I assume that what you are studying is talking about using one of these Private networks. So you might use network 172.16.0.0, which is a class B network, and subdivide it to create the subnets. You might configure the first subnet as 172.16.0.0 255.255.255.0 and assign it to vlan 10. You might then configure the second subnet as 172.16.1.0 255.255.255.128 and assign it to vlan 20. The concept of subnet zero and subnet one is just a way of numbering the subnets where the numbering starts with zero (many of us would probably think when we are numbering something that it starts with one, but IP subnetting starts the numbering at zero)

- "2) And could you explain "..associate interfaces 6 thru 12 with VLAN 10 and associate interfaces 13 thru 18 with VLAN 20" It might be helpful if we knew more about the context of this statement. I am guessing that it is talking about configuring a switch and on that switch interfaces 6 through 12 are assigned to vlan 10 and interfaces 13 through 18 are assigned to vlan 20. You would configure each interface as an access interface and assign its vlan membership which might look something like this

interface Gig1/6

switchport mode access

switchport access vlan 10

[Anonitachi]

Rick, 

IO had figured out/read-up on, the switchport and used range to include a number of ports ex: int range fa0/19-20.

In regards to the 172***, totally my fault on that one. It wouldn't register in my brain so many thanks for clarifying.

I will bookmark this thread for reference and thanks to you @martin as well as yourself @rick for the prompt input.

Take care, and I'm sure I'll need you(s) again at some point.

Cheers

[Richard Burts · Accepted solution]

This has been a good discussion and I am glad that you found it useful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

I remember when I was starting in networking and thought that subnetting and VLSM were quite challenging. With experience I now find it much easier. I trust that you will have the same experience. This community is a very helpful resource and I encourage you to use it freely.

[neilneil]

Hi all, 

I seem to have been having the same problem but these have made a bit more sense of it for me. As i understand it, subnet 0 is for VLAN 10 and subnet 1 is for VLAN 20, but how are the additional subnets of 16 created?

For some reason i do not understand what is required of me, do i need to use a mask of /27. I hope i have explained what i'm looking for clearly enough.

Thank you all for the help i have seen you have given already.

Neil

[Georg Pauwen]

Hello,

since this is a rather old (and long) post, can you briefly summarize and restate your problem ?

[neilneil]

Hi there, thanks for your response.

I have to create 2 sub interfaces for a router for VLANs 10 and 20. There are 2 interfaces on the router, one with an IP address of 99.0.0.1 255.0.0.0 and another interface with no ip address. 

Am i correct in assuming the new sub interfaces are configured on the interface with no ip address, if so, how do find out the correct ip address?? I need to enable dot1q for these as well, but i understand that part.

The next part says "Using VLSM, split 172.16.0.0/24 into 16 further subnets. VLAN being subnet 0 etc. I need to create suitable ip addresses and subnet masks to reflect this configuration.

Im very grateful for any help received.

Neil

[Richard Burts]

This is an old thread that Neil has adopted. So thank you for some clarification of what you are trying to do. Given the information that you provided it would make sense to configure the 2 vlans as subinterfaces on the interface that has no ip address. And yes these subinterfaces would use dot1q.

You ask "if so, how do find out the correct ip address?" We do not have information to be able to suggest answers for this. Assuming that this is some type of assignment/project, was there any more information about these 2 vlans/subnets?

If you want to split 172.16.0.0/24 into 16 further subnets the easy thing would be to create 16 subnets each of which would have 16 addresses. I am not clear how significant it is that the instruction used the term VLSM. If you create 16 subnets and each one has 16 addresses (14 usable addresses + subnet address+ broadcast address) that is not very variable. Perhaps create one subnet with 32 addresses and 2 subnets with 8 addresses to get variable subnets?

[neilneil]

Hi Rick,

Thanks for taking the time to respond. I think the biggest problem I have been having is that the assignment requires 172.16.00/24 to be split using VLSM, however, there are no details about numbers of hosts. You have been helpful and I think I need to get clarification from the tutor regarding the VLSM issue.

The instructions i received are attached.

Thankyou,

Neil

[Richard Burts]

Neil

The part of the instructions that you post say to see above selection method. What selection method is that?

Assuming that there were instructions about creating vlan 10 and 20 were there any instructions there about IP addressing for those vlans? That might provide a clue about what IP addresses to us for the router subinterfaces?

These instructions seem to be saying that vlan 10 and 20 would use the first two subnets when you subdivide 172.16.0.0. So perhaps this is the clue about what IP addresses to use for the vlan subinterfaces on the router?