Stuck in a LAB

QUESTION
Refer to above network diagram
You are the network administrator at Ranet, and
have to config the router "Ranet-BR" via Console
Terminal as below:
Task 1. enable interface Se0/0/0 and set IP address
/Subnet Mask that can connect with router
"Ranter-HQ" at 192.168.0.254/30
Task 2. enable interface Se0/1/0 and set IP address
/Subnet Mask that can connect with router
"Ranter-BR1-1" at 192.168.0.30/30
Task 3. The Headquater has given IP address
range 192.168.0.0/27 for the network
behind our router, then you have to:
3.1 enable interface Fa0/0 and set
IP address/Subnet Mask to be the
last IP of this subnet that has
10 host as members.
3.2 enable interface Fa0/1 and set
IP address/Subnet Mask to be the
last IP of this subnet that has
5 host as members.
3.3 Set IP address/Subnet Mark of Host1
in each LAN to be the first IP of its
own subnet, and set IP of gateway also

If the configuration is correct, Host1 in each subnet
must be able to ping to Ranet Server at 192.168.0.33

ANSWER

I have completed Task 1 and 2 by giving Se0/0/0 "192.168.0.253 255.255.255.252" and Se0/1/0 "192.168.0.29 255.255.255.252"
Problem is with 3.1,3.2,3.3
The solution for 3.1 and 3.2 says giving 192.168.0.14/28 to Fa0/0 and 192.168.0.22/29 to Fa0/1 .What I dont understand is that why they have used ip address range 1-14(as 0 and 15 cant be used) and address range 17-22(as 16 and 23 cant be used) of all the subnets available??? 3.3 is straight forward no problem with it as 3.1 and 3.2 ranges would be utilized here.

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Comments

spd3432

What don't you understand about it? Simple subnetting using VLSM (variable length subnet masks).

Diggs

What other ranges do you think they should have used?

hassantalal785

I have done all the sub netting and i am well familiar with VLSM . (For 3.1) I know the sub nets would be 0-15 , 16-31, 32-47 and so on as Block size is 16. What i don't understand is that why the 1st sub net i.e from 192.168.0.1 to 192.168.0.14 (as 192.168.0.0 and 192.168.0.15 are invalid ip address for Host) is used ? (For 3.2) Block size is 8 so subnets are 0-7 , 8-15 , 16-23 ....... why third sub net i.e 192.168.0.17 to 192.168.0.22 (as 192.168.0.16 and 192.168.0.23 are invalid ip address for Host) is used ?

spd3432

I don't think you truly understand VLSM. You only have a given range of addresses available to you.

You were GIVEN the following address:
192.168.0.30/30 as the interface on Ranet-BR1-1.
Therefore
192.168.0.29/30 MUST be the connecting interface on Ranet-BR.
192.168.0.28/30 MUST be the network address for that link.
192.168.0.31/30 MUST be the broadcast address.

Therefore, all other IP usage by you MUST stay under 192.168.0.27.

3.1 -- you have to accomodate 10 hosts in LAN1. The smallest usable block of addresses that can be used to meet the 10 host requirement is a block of 16 (/28 mask)
192.168.0.0/28 <-- network address
192.168.0.1/28 <-- first usable address
192.168.0.14/28 <-- last usable address
192.168.0.15/28 <-- broadcast address

3.3 -- you have to accomodate 5 hosts in LAN2. The smallest usable block of addresses that can be used to meet the 5 host requirement is a block of 8 (/29 mask). You CAN'T use /28 again because that would put your last host at 192.168.0.30 with broadcast on 192.168.0.31 and those are already in use.

192.168.0.16/29 <-- network address (next number up from your /28 broadcast)
192.168.0.17/29 <-- first usable address
192.168.0.22/29 <-- last usable address
192.168.0.23/29 <-- broadcast address

192.168.0.24/30 network is not used in this scenario. 192.168.0.25/30 and 192.168.0.26/30 would be available for use only for another point-to-point link at some time in the future.

hassantalal785

spd3432 wrote: »

Therefore, all other IP usage by you MUST stay under 192.168.0.27.

.

First of thanks a lot for such a detailed answer. It was really helpful in clearing my concepts.Kindly elaborate a further when u say "other IP usage by you MUST stay under 192.168.0.27". I myself answered the following correctly that :
192.168.0.29/30 MUST be the connecting interface on Ranet-BR.
192.168.0.28/30 MUST be the network address for that link.
192.168.0.31/30 MUST be the broadcast address.

Is it because HQ has said that 192.168.0.0/27 is given to network behind router Ranet-BR1-1 as for 192.168.0.0/27 will have a block size of 32 and its subnets would be :
192.168.0.0 ---- 192.168.0.31
192.168.0.32 ---- 192.168.0.63
192.168.0.64 ---- 192.168.0.95
and so on ........
and we have to remain in first sub net as router Ranet-BR1-1 's interface to Ranet-BR has 192.168.0.30 address ?

spd3432

Yes, we have to remain in the first subnet, but not because of the interface IP on Ranet-BR1-1. We have to remain in the first subnet of 192.168.0.0/27 (IP range 192.168.0.0 --> 192.168.0.31) because that's what the network design calls for. Someone at corporate is responsible for the entire network and all IP ranges. They've made the decision as to which ranges will be used in which locations. We have to stay under 192.168.0.27 because we've already used the numbers above that which were allocated to this site (192.168.0.28 -> 192.168.0.31).

hassantalal785

Thanks indeed .Its crystal clear now