Determine the Internet addresses that describe each of the 3 subnets using FLSM

Question

Determine the Internet addresses that describe each of the 3 subnets using FLSM Determine the Internet addresses that describe each of the 3 subnets using VLSM. You must present the following information for each one of the 3 assigned subnets in the two scenarios (6 in total): Network address First usable address of the subnet Last usable address of the subnet Broadcast address for the subnet Prefix length Subnet mask You may consult the example that is provided under this Week’s materials. Please explain any assumptions or calculations you made to arrive to your final solution.. Please write your IP range and network sizes in the subject Line. Or at least, write them in the first lines of your posting. This way we will not have to go back to the top of the discussion thread to see if your calculations were done correctly. Also, please present one table for FLSM and one table for VLSM using the following format, this will help everyone to read your calculations: Subnet # Prefix Length Subnet Mask # Required Hosts Max # of Hosts Network Address First Address Last Address Broadcast Address 1 response to other students’ postings by Sunday 11:59pm Central Time deadline (Notice this week only 1 response to other student posting’s is needed). This response should evaluate the calculations made by the other student and confirm the solution or provide a suggestion on how to correct or improve the solution. Please evaluate a student that has not been evaluated before and do not post calculations for the other student! But you may suggest them. Let them do her/his own work. You may recommend them what this work is. 17.1.124.0/22 70 120 2

Explanation / Answer

VLSM is a process of dividing an IP space into the subnets of different sizes without wasting IP addresses. When we perform subnetting, all subnets have the same number of hosts, this is known as FLSM ( Fixed length subnet mask). In FLSM all subnets use same subnet mask, this lead to inefficiencies. In real life scenario, some subnets may require large number of host addresses while other may require only few addresses.

For example, assume that you are a network administrator at Laxmisoftwares. Company have three departments connected with wan links.

With FLSM, to accumulate this requirement you have two choices, either purchase a class B IP address space or purchase at least two class C IP address space.

First choice with a example class B address space

172.168.1.0/23

Subnetting of this address space would give us 128 subnets and 510 hosts in each subnet. Our network requires only 6 subnets and 160 addresses. Every IP address add more dollars in company bill. You would have to pay for 65356 addresses while you need only 160 addresses. Would you consider this address space for company?

Second choice with two example class C address spaces

Subnetting of first address 192.168.1.0/25 would give us 2 subnets and 126 hosts in each subnet.

Subnetting of second address 192.168.2.0/26 would give us 4 subnets and 62 hosts in each subnet.

Collectively we are getting 6 subnets and 500 hosts from these two address spaces. We are still wasting more than 300 IP address, and we would have to purchase two address spaces.

Variable Length Subnet Mask

Variable Length Subnet Mask (VLSM) extends classic subnetting. VLSM is a process of breaking down subnets into the smaller subnets, according to the need of individual networks. In above example company have requirement of 6 subnets and 160 host addresses. With VSLM you can fulfill this requirement with single class C address space.

VLSM Subnetting

In VLSM Subnetting, we do subnetting of subnets according the network requirement.

Steps for VLSM Subnetting

VLSM Example

Now you know the steps of VLSM Subnetting. Let's understand it with above example. Our company requires 6 subnets and 160 hosts.

Step 1 :- Oder all segments according the hosts requirement (Largest to smallest).

Step 2 :- Do subnetting for largest segment. Our largest segment needs 74 host addresses. /25 provide us two subnets with 126 hosts in each subnet.

192.168.1.0/25

Step 3 :- Assign subnet mask to the largest segment. As you can see in above table, subnet 1 fulfill our largest segment requirement. Assign it to our segment.

Step 4 :- Do subnetting for second largest segment from next available subnet. Next segment requires 52 host addresses. Subnetting of /25 has given us two subnets with 126 hosts in each, from that we have assigned first subnet to development segment. Second segment is available, we would do subnetting of this.

/26 provide us 4 subnets with 62 hosts in each subnet.

192.168.1.0/26

We cannot use subnet 1 and subnet 2 ( address from 0 to 127 ) as they are already assigned to development department. We can assign subnet 3 to our production department.

Step 5 :- Our next segment requires 28 hosts. From above subnetting we have subnet 3 and subnet 4 available. Do subnetting for the requirement of 28 hosts.

192.168.1.0/27

Subnets 1 to 6 [ address from 0 to 191] are already occupied by previous segments. We can assign subnet 7 to this segment.

Step 6 :- Our last three segments require 2 hosts per subnet. Do subnetting for these.

192.168.1.0/30

Valid subnets are:-

0,4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76,80,84,88,92,96,100,104,108,112,116,120,124,128,132,136,140,144,148,152,156,160,164,168,172,176,180,184,188,192,196,200,204,208,212,216,220,224,228,232,236,240,244,248,252,256

From these subnets, subnet 1 to subnet 56 ( Address from 0 - 220) are already assigned to previous segments. We can use 224,228, and 232 for wan links.

Assign these subnets to wan links.

Wan Link 1

Wan Link 2

Wan link 3

We have assigned IP addresses to all segments, still we have 20 addresses available. This is the magic of VLSM.

Classful and classless, these two terms are also used for FLSM and VLSM.

Classful subnetting

FLSM is also known as classful subnetting as all subnets have same number of hosts. In classful subnetting all subnets use same subnet mask.

Classless subnetting

VLSM is also known as classless subnetting as all subnets may have different number of hosts depending upon network requirement.

Classful routing

RIPv1 and IGRP routing protocols do not have a field for subnet information. It means that if a router running RIP routing protocol, has a subnet mask of a certain value, it assumes that all interfaces within the classful address space have the same subnet mask. This is known as classful routing.

Classless routing

RIPv2, EIGRP and OSPF are known as classless routing protocols, as they have field for subnet information in their routing advertisement. VLSM only works with classless routing protocols.

Subnet Segment Hosts 1 Development 74 2 Production 52 3 Administrative 28 4 Wan link 1 2 5 Wan link 2 2 6 Wan link 3 2

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