topic Re: Overlaps with vlan in Switching

Overlaps with vlan

vishmithathejan154 — Wed, 29 Jan 2025 13:33:16 GMT

Hello, I'm new to networking trying to configure Layer 3 Switch. I have calculated subnets for each vlan

```Vlan 10 - 2 users
200.100.10.0/29
200.100.10.1
200.100.10.2-3
200.100.10.4

vlan 20 - 10 users
200.100.10.5/28
200.100.10.6
200.100.10.7-16
200.100.10.17

vlan 30 - 29 users
200.100.10.18/26
200.100.10.19
200.100.10.20-48
200.100.10.49

vlan 40 - 25 users
200.100.10.50/27
200.100.10.51
200.100.10.52-76
200.100.10.77

vlan 50 - 3 users
200.100.10.78/29
200.100.10.79
200.100.10.80-82
200.100.10.83

vlan 60 - 29 users
200.100.10.84/27
200.100.10.85
200.100.10.86-114
200.100.10.115

vlan 70 - 15 users
200.100.10.116/27
200.100.10.117
200.100.10.118-132
200.100.10.133

vlan 80 - 4 users
200.100.10.134/29
200.100.10.135
200.100.10.136-139
200.100.10.140

vlan 90 - 12 users
200.100.10.141/28
200.100.10.142
200.100.10.143-154
200.100.10.155

vlan 100 - 4 users
200.100.10.156/29
200.100.10.157
200.100.10.158-161
200.100.10.162
```
This is my configurations!

```KL3sw#conf ter
KL3sw#conf terminal
Enter configuration commands, one per line. End with CNTL/Z.
KL3sw(config)#inter vlan 10
ip address 200.100.10.1 255.255.255.248
KL3sw(config-if)#no shut
KL3sw(config-if)#exit
KL3sw(config)#inter vlan 20
KL3sw(config-if)#ip address 200.100.10.8 255.255.255.240
% 200.100.10.0 overlaps with Vlan10
KL3sw(config-if)#ip address 200.100.10.9 255.255.255.240
% 200.100.10.0 overlaps with Vlan10
KL3sw(config-if)#ip address 200.100.10.10 255.255.255.240
% 200.100.10.0 overlaps with Vlan10
KL3sw(config-if)#ip address 200.100.10.17 255.255.255.240
KL3sw(config-if)#ip address 200.100.10.16 255.255.255.240
Bad mask /28 for address 200.100.10.16
KL3sw(config-if)#ip address 200.100.10.14 255.255.255.240
% 200.100.10.0 overlaps with Vlan10
```
Why I can't configure vlan 20?

PS: I have configured DHCP server on a server according to my subnet calculations.

Please help me!

Re: Overlaps with vlan

Joseph W. Doherty — Wed, 29 Jan 2025 14:36:25 GMT

Your subnetting calculations are incorrect. Do you need further help to calculate them correctly? (Hint: if they are not a power of 2, they are incorrect.)

Re: Overlaps with vlan

vishmithathejan154 — Wed, 29 Jan 2025 14:41:21 GMT

Can you help me by using example? Only if you can. So I can learn...

Re: Overlaps with vlan

Flavio Miranda — Wed, 29 Jan 2025 14:59:01 GMT

@vishmithathejan154

You first subnet should be

``Vlan 10 - 2 users
200.100.10.0/30
200.100.10.1-2
200.100.10.3

That´s why the device is refusing vlan 20

Re: Overlaps with vlan

Richard Burts — Wed, 29 Jan 2025 19:48:00 GMT

The OP design has 200.100.10.0/29. With that subnet mask the vlan would use addresses beginning at 200.100.10.0 and going through 200.100.10.7. @Flavio Miranda makes a correct point that the mask could actually be smaller. But even if you change the mask there would still be a problem.

The problem is that vlan 20 uses a /28 mask. And a /28 mask provides 16 addresses (14 usable) and the subnets would start at 0, then 16, then 32 etc. So the ip address of 200.100.10.6 falls into the same address space as vlan 10.

The OP asks "Why I can't configure vlan 20?". The answer is that you can configure vlan 20 you just have to make sure that the addresses it uses do not overlap with existing subnets. And in fact this command ip address 200.100.10.17 255.255.255.240 did successfully configure vlan 20.

Re: Overlaps with vlan

Richard Burts — Wed, 29 Jan 2025 22:33:12 GMT

A couple more thoughts about this:

This is interesting ip address 200.100.10.16 255.255.255.240
Bad mask /28 for address 200.100.10.16

The mention of Bad mask is because the .16 address is the network address of the next subnet/next vlan and can not be used as a host address.

I stated that vlan 20 had been successfully configured and I stand by that. But it is not configured with the addresses identified in the OP. In designing and configuring vlans and subnets you need to be very careful about what mask is being used and what the first and last usable address of the subnet will be.

Re: Overlaps with vlan

Joseph W. Doherty — Thu, 30 Jan 2025 00:33:57 GMT

@vishmithathejan154 wrote:

Can you help me by using example? Only if you can. So I can learn...

Sure, although this is now going to overlap with others' replies.

Networks are binary multiple sizes, i.e:

1 = /32

2 = /31

4 = /30

8 = /29

...

256 = /24

and so on.

Addresses, for any particular network, must start on a multiple of that size, for example, given the addresses 0 through 7, a /32 can be any of those; a /31 must be 0, 2, 4, 6; a /30 must be 0 or 4; a /29 can only be 0.

If you allocate any network, it cannot be part of a larger network that would encompass the same address(es). So, for example, if you allocate the /29 for 0..7, you can not allocate any of the smaller networks within 0..7 or conversely, if you allocate any smaller network from 0..7, you cannot use the /29 that uses them.

As network allocations require specific boundaries, you cannot always just allocate the next starting address to a larger address block.

For example, if you allocate 0 and 1 to a /31, you cannot allocate the next starting address, 2, to a /30, because it must start with a multiple of 4. Yet you could allocate 2 and 3 to a /31, and then allocate the next starting address, 4, to the /30, because 4 is a multiple of 4.

You likely know, the first address and last address are not used for host addresses. /31s can be used for p2p links, and /32s can be used both as a host address or network.

Also confusing, 192.168.1.0..7/29 addresses are all within the 192.168.1.0/29 network.

What I wrote about 0..7 can be repeated for 8..15, although now we can have 0..15 as a /28 network.

If VLAN 10 is to support just two users, it would normally, minimally, be assigned a /30 (as @Flavio Miranda showed), addresses 0..3 (2 hosts), but you could use the larger /29, addresses 0..7 (6 hosts).

If VLAN 20 is to support 10 users, it would normally, minimally, be assigned a /28 (14 hosts), but it must start on a multiple of 16, i.e. it cannot start directly after the foregoing /30 or /29, i.e. with either .4 or .8, so it would be 16..31.

To proceed with your other VLAN allocations, you need to understand VLAN 10 and VLAN 20 allocations.

BTW, whether VLAN 10 is allocated as a /30 or /29, it will leave some unused address space between it and VLAN 20's /28. That unused space could be allocated to support other small VLANs, as either /30 (V10), /30, /29, /28 (V20) or /30 (V10), /30, /30, /30, /28 (V20) or /29 (V10), /29, /28 (V20) or /29 (V10), /30, /30, /28 (V20).