VLSM related question

colin300

Right this should be a quick enough question. I will write out the question and how i did it so you dont have to go through it when your trying to answer the question.

First of all i am given a network ID of 10.150.25.0/24 (bear in mind i am using vlsm for this). There are 3 offices. Main office needs 100 IPs and the other 2 need 30 IPs.

So I subnet it to a /25 and use 10.150.25.0/25 for the main one we shall call it LAN A.

I then subnet 10.150.25.128/25 to a /27 and get 10.150.25.128/27, 10.150.25.160/27, 10.150.25.192/27.

I use .128 and .160 for LAN B and LAN C.

Each LAN is connect to the other by a router and they have a leased lines going between each. So A connects to B. A connects to C. B connects to C.

In order for these to communicate i further subnet down 10.150.25.192/27 to a /30 which makes 10.150.25.192/30, .196/30, .200/30 and so on but i only need them for the routers. The routers between LAN A and B are given the subnet of .192/30. The routers between LAN A and C are given the subnet of .196/30. The routers between B and C are given the subnet of .200/30.


Now the question is the following. Is 10.150.25.0/25 a valid network and why is it a valid network since .0/25 would mean it is given the network ID?

The next one is: Apart from the network ID and the broadcast address of each subnet is there any other reserved subnets like there is in 'classful'?

If you were going to use RIPv2 that supports VLSM would it recognise /25 and see they are not the /24 (i.e. the main network ID)?



Now in what i have written i believe beyond all shadow of doubt that i am 100% correct in what i have written i.e. the subnetting. But I need a place that describes the rules and how VLSM uses this because believe it or not my teacher believes I am wrong and that aint really good when you know your right and he is teaching the class like this.
Plus an example in Applied Data Communications: A Business Oriented Approach makes him believe further that i am wrong because while using vlsm he disregards the first and last subnet after creating 8 subnets which is completely wrong.

dingding

The newer cisco ios's allow you to use subnet zero. Previous versions of the ios from a couple of years ago do not allow the use of subnet zero. you need to enter a command into the router, i think it is ip subnet zero

lectuters teaching the ccna curriculum sometimes ignore subnet zero for simplicity.

keep up the networking.

colin300

so what you are saying is when VLSM was created with the use of all the IPs in mind the .0/25 subnet say was disregarded (i.e. .1 all the way up to .126)? and sdont forget since .128/25 is the last subnet this would be disregarded aswell.

But they created VLSM to stop this wastage not make more of it? It is irrelevant if cisco didn't allow it or not i am just talking about what VLSM is there for.

dingding

Cisco's implementation of VLSM which is proprietary allows you to use the first subnet and not the last subnet. normally you can not use either the first or last subnet. I suppose technically your solution would be wrong but you could implement in some of the newer cisco ios's.

colin300

Right so would it be usable in CIDR?