VLSM Subnetting

TallGlass

Ok I am having a bit of troble for some reason wrapping my head around the using VLSM to sub-net bigger blocks of addresses.

I will show you the diagram I was trying to work out before I found the answer online. Even then I still couldn't work it out. For some reason my brain will only allow me to sub-net the last octet up IE if the mask is over the /24 but anything below /24 confuses the hell out of me, I think I am having trouble just trying to figure out where the sub-nets would end and another one will start.

http://www.hh.se/download/18.70cf2e49129168da0158000102209

Could someone explain how to subnet the bigger hosts address requirements? IE 8000/4000/2000/1000 500 hosts per subnet and how to correctly find the next subject network address. Or even explain the diagram!

Cheers.

65535

The best tool I have found for this is on Android called 'IP Network Calculator' where you can increase or decrease the number of hosts using a sliding scale and it calculates all the relevant address information for you there and then.

If you think of a link between two routers
They only need 4 addresses.
1 address for each router, 1 address for broadcast and 1 for Network ID
For example
Network: 192.168.0.0
Broadcast: 192.168.0.3
Netmask: 255.255.255.252
First Address: 192.168.0.1
Last Address: 192.168.0.2
Number of useable IP's: 2
The CIDR (Classless InterDomain Routing) notation for that subnet is /30

See also http://doc.m0n0.ch/quickstartpc/intro-CIDR.html

TallGlass

Thanks for the help 65535, the problem wasn't the links but more how to plan the implementation of the network addressing on a large scale.

I asked one of the guys in work, he had a bit of an idea and I basically answered my own question. With VLSM on a large scale such as the diagram I posted. I came to the following;

1. Add up all the hosts in each area IE Central West East etc..

So it would look something like

=> Central 16000 hosts total
=> West 8000 hosts total
=> East 4000 hosts total

Find out the powers of 2 and get closest match and figure out the bits

Then VLSM the address that is given EG 192.168.0.0

Central would get the range;
192.168.0.0 - 192.168.63.255 Mask 255.255.192.0 (64)

West would get the range;
192.168.64.0 -192.168.95.255 Mask 255.255.224.0 (32)

East would get the range;
192.168.96.0 - 192.168.113.0 Mask 255.255.240.0 (16)

Then I would just VLSM each subnet according to the requirements within the networks itself. Is this allowed? Or would it be an overlapping subnet, I am think no. Because the ranges are just ranges and wouldn't be assigned as such to any interfaces. I just put the masks in for my own logic and to show how I go the next subnet.

Would this be correct?

ED E

You can always subdivide a /22 into two /23s and so on, just be careful that you apply the offsets for increased bcast and network addresses.

TallGlass

ED E wrote: »

You can always subdivide a /22 into two /23s and so on, just be careful that you apply the offsets for increased bcast and network addresses.

Hi Ed E. Yes this is what I was thinking. But two questions.

1. If I subbnetted /23 and the /22 is it a case the subnets are then overlapping.

2. Could I subnet like so;
Network side A /17 /18/19/30
Network side B (starting at the next subnet after /30) /18/19/20/30

ED E

=> Central 16000 hosts total
=> West 8000 hosts total
=> East 4000 hosts total

Central gets a /18 at minimum*
West gets a /19 at minimum*
East gets a /20 at minimum*

Central: 192.168.0.0 - 192.168.63.255
West: 192.168.64.0 - 192.168.95.255
East: 192.168.96.0 - 192.168.111.255

Say then we want to divide Central into 4 floors:
Take the /18 and make 4x /20s
CentralF1: 192.168.0.0 - 192.168.15.255
CentralF2: 192.168.16.0 - 192.168.31.255
CentralF3: 192.168.32.0 - 192.168.47.255
CentralF4: 192.168.48.0 - 192.168.63.255 - Still doesnt overlap with next network

You can never have overlap, and when you split you always split "symmetrically", then subdivide things out from there. You can use excel to map it out if needed

* If you can, leave room for expansion so you dont have a huge headache later when department x expands.