Question

Question 11 The IP address for the above network is 192.168.151.0/24 (a) What is the total number of networks in the diagram above? (b) Each individual internal network (the networks behind the routers) must support a maximum of 13 User PCs. Can this be done without using VLSM? Why? Write briefly how VLSM can be used in this case to achieve the required conditions? (You do not need to write out the IP addresses for the individual subnets) (c) (3 marks)

Answer 1

a) The total number of networks is 19.

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b)

Without VLSM, the network can't satisfy the specified conditions. Since there are a total of 19 networks, FLSM is not possible as we need 15 IP's for each internal network which requires a total of 19*15=285 IP's. But, we have a maximum of 256 IP's. FLSM can't be possible here.

S.No	Actual IP's	Required IP's	Total IP's
N1 to N19	13 user PC's	2 additional (one for n/w IP + one for broadcast IPs)	13+2=15

c)

With the help of VLSM, we can achieve the above network with ease. We can reduce the wastage of IP's in Variable Lenght Subnet Masking(VLSM). We can assign IP's as per our requirement.

Our block size can be greater than or equal to our host's requirement.

Out of 19 networks, we have 9 internal networks which support 13 User PCs, i.e., we need a total of 15*9=135 IPs.

9 Subnets with block size 16. They are 0-15,16-30,31-45,46-60,61-75,76-90.91-105,106-120,121-135.

For the remaining 10 networks, serial connections we need 4 hosts each i.e., 10*4=40 IPs. The block size is 4 for serial connections. They are 136-139,140-143,---and so on.

SNo	Actual IP's	Required IP's	Total	Block Size
N1 - N9	13 User PC's	n/w IP + broadcast IP	15	16
N10 - N19 (Router-Router Connections)	2	2	4	4