25-03-2011, 12:25 PM
CCNA3 v3.1 Module 1.ppt (Size: 1.82 MB / Downloads: 61)
Objectives
What Is VLSM and Why Is It Used?
Variable-Length Subnet Masks
Variable-Length Subnet Masks (VLSM)
VLSM allows an organization to use more than one subnet mask within the same network address space.
Network administrator can use:
a long mask on networks with few hosts, and a short mask on subnets with many hosts;
a routing protocol that supports VLSM;
Classful Routing
Classful routing protocols
require that a single network use the same subnet mask.
Classless Routing
VLSM
allows a single autonomous system to have networks with different subnet masks.
VLSM
VLSM
Calculating VLSMs
Waste of Space
In the past:
it has been recommended that the first and last subnet not be used;
use of the first subnet (subnet zero), for host addressing was discouraged because of the confusion that can occur when a network and a subnet have the same addresses.
Now:
It has become acceptable practice to use the first and last subnets in a subnetted network in conjunction with VLSM.
Waste of Space
If the last three subnets are used for the WAN links, all of the available addresses will be used and there will be no room for growth;
also have wasted the 28 host addresses from each subnet to simply address three point-to-point networks;
this address scheme would waste one-third of the potential address space.
Waste of Space
use subnet zero;
there will be eight useable subnets;
each subnet can support 30 hosts.
“IP subnet-zero” Command
no ip subnet-zero command
From Cisco IOS version 12.0, Cisco routers use subnet zero by default.
When to Use VLSM
Using VLSM
Large subnets are created for addressing LANs;
Very small subnets are created for WAN links and other special cases;
A 30-bit mask:
- used to create subnets with only two valid host addresses;
- the best solution for the point-to-point connections.
Sub-subnets
Calculating Sub-subnets
VLSM
allows for the setting of a subnet mask that suits the link or the segment requirements;
should satisfy the requirements of a LAN with one subnet mask and the requirements of a point-to-point WAN with another.
Calculating Sub-subnets
Classful Addressing Scheme
Classful routing protocol RIP v1, IGRP, and EGP
do not support VLSM;the WAN link must be a subnet of the same Class B network;the WAN link would need the same subnet mask as the LAN segments; а 24-bit mask of 255.255.255.0 can support 250 hosts.
VLSM Example
VLSM – can be used to subnet an already subnetted address
Example - subnet address 172.16.32.0/20 and network that needs ten host addresses
Using 172.16.32.0/20:
- there are 2^12 – 2, or 4094 host addresses, most of which will be wasted.
Using VLSM:
it is possible to subnet 172.16.32.0/20 to create more network addresses with fewer hosts per network;
172.16.32.0/20 is subnetted to 172.16.32.0/26:
- there is a gain of 2^6, or 64 subnets;
- each subnet can support 2^6 – 2, or 62 hosts.