CCNA 640-802 Study Note–VLSM Revisit

As mentioned in one of my post, Variable-Length Subnet Mask (VLSM) can help to have a more efficient use of IP address. We will use a simple example to look at why is this the case.

Imagine you were given a subnet 192.168.10.64/26. With this subnet, you can have up to 62 hosts (2⁶ – 2). Now you need to have 4 VLANs for the Sales, IT, HR and Finance departments. Each department will have at most 10 hosts and 1 IP for the router interface. One option is to ask for more subnets (e.g. 192.168.10.0/26, 192.168.10.128/26 and 192.168.10.192/26). However, with this option, you will be wasting 51 hosts IP for each subnet. Okay, you asked for more subnets and was rejected.

“Sorry, we are running out of P addresses. Please make do with what is given. Thank you.”

You stared hard at the subnet 192.168.10.64/26. Suddenly you thought of VLSM and came out with the following.

Network: 192.168.10.64/28
Host Range: 192.168.10.65 - 192.168.10.78

Network: 192.168.10.80/28
Host Range: 192.168.10.81 - 192.168.10.94

Network: 192.168.10.96/28
Host Range: 192.168.10.97 - 192.168.10.110

Network: 192.168.10.112/28
Host Range: 192.168.10.113 - 192.168.10.126

Hooray, you just managed to achieve what you need by further subnetting a subnet. You have 4 subnets with each subnet giving you 14 hosts. You have saved 204 IP addresses!

Below is a simplified snapshot of what you have just achieved.

SwitchA#sh vlan brief

VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0/5, Fa0/6, Fa0/7, Fa0/8
Fa0/9, Fa0/10, Fa0/11, Fa0/12
Fa0/13, Fa0/14, Fa0/15, Fa0/16
Fa0/17, Fa0/18, Fa0/19, Fa0/20
Fa0/21, Fa0/22, Fa0/23, Gig1/1
Gig1/2
2 Sales active Fa0/1
3 IT active Fa0/2
4 HR active Fa0/3
5 Finance active Fa0/4
1002 fddi-default active
1003 token-ring-default active
1004 fddinet-default active
1005 trnet-default active

This is really a very simple example. When this small little network gets connected to other networks which requires exchanging of routing information, we will need a routing protocol that can support VLSM. The 3 routing protocols covered in CCNA that support VLSM are RIPv2, OSPF and EIGRP.

If you feel like playing around with this setup, you can download the Packet Tracer Network File from http://www.mediafire.com/?26p52w69hbwr92x.

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