IP Overlap between vlans

[BryceBurbank32897]

Hey, I need to IP my vlans on a Cisco WS-3750X-48P, but I am getting an IP overlap problem. I have the ips and subnets written out and maybe I just need a sanity check because I just cannot seem to get it to work.

Vlan 10: 

ip address 192.168.1.1 255.255.255.252

.0 should be Net ID, .1-.2 should be usable, and .3 should be broadcast.

When I attempt Vlan 20 with

ip address 192.168.1.5 255.255.255.240

I get

192.168.1.0 overlaps with Vlan 10

Which is very confusing because this range should have .4 as Net ID, .5-.18 as usable, and .19 as broadcast.

I'm not exactly sure what I am doing wrong here, so any help would be very appreciated!

[Jon Marshall · Accepted solution]

You can't just start at the next available IP, it does not work that way. 

So for 192.168.1.5 255.255.255.240 the actual subnet is 192.168.1.0 255.255.255.240 which is why you have an overlap. 

You should really do it all in binary until it makes more sense but as a quick guide to work out the subnets in use with a certain mask take the first octet in the subnet mask that is not 255 so in this case that would be 240 and - 

256 - 240 = 16 which means your subnets with a 255.255.255.240 mask go up in 16s ie. 

192.168.1.0 

192.168.1.16

192.168.1.32 

etc. 

so you can hopefully see that if you use 192.168.1.0 255.255.255.252 then the next available subnet using a 255.255.255.240 is 192.168.1.16. 

Hope that makes sense. 

Jon

[BryceBurbank32897]

Ip'ing the second vlan as to 192.168.1.17 did work

I guess I am a bit confused as to why a /30 subnet that should only be taking 4 addresses (.0-.3) overlaps with the /28 which I would like to be .4-.19

I guess what I am trying to ask is what exactly are the addresses for .4-.15 being used for now?

[Jon Marshall]

They aren't being used unless you allocate them. 

If you stick with your scheme you could use those IPs as follows - 

using a /30 mask for all subnets - 

192.168.4.4/30

192.168.4.8/30

192.168.4.12/30 

or you could use a /30 and a /29  ie. 

192.168.4.4/30 

192.168.4.8/29 

again to see why you have to use them like this use the method I showed you and it should become more obvious. 

As mentioned usually you start with the largest subnets first and then use the smaller ones later. 

Feel free to ask as much as you like, subnetting is one of those things that can take a while before it becomes second nature to do. 

Jon

[Martin L]

Yes, because smaller range of /30 falls into a bigger one, /28. and they all fall into /24. let me find Magic number to help u out.

Principles of VLSM are: 

1. Start with the larger block/network (host requirement).

2. is to take larger block and split it into smaller ones.

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

[BryceBurbank32897]

Oh my gosh, I see now. That make sense. Thank you so much

[Martin L]

If you study for CCNA exam, memorize Magic number table and write it out before actually starting exam (during 15 min of intro). it will help u to verify your calculations or to do class C subnetting. Note: you still must learn subnetting by hand. Use table to double check.