Please Help Me On VLSM, Subnetting, and Cisco Router

kage

I have a skills final on Monday in my Cisco Fundamentals of Networking class and I need help or tips on VLSM. I'm finally starting to understand subnetting. Also I would like to have some tips on configuring the Cisco Router.

EdTheLad

VLSM and subnetting?? I'm sure i've seen them mentioned before in this forum, maybe you should try the seach engine.

daniel2009

I was wondering about the same thing yesterday. It has to do with subdivisions of the networks.
When ARPANET started, they decided the army should have 256 networks.
Thus:
255.0.0.0

the first byte is used as network

When companies started to have interest in ARPANET, they needed to have more networks. They figured it can be further divided if using the 2nd and 3rd byte for hte networks.

Copanies could buy either a Class A, Class B or Class C network.

Class A 255.0.0.0
Class B 255.255.0.0
Class C 255.255.255.0

This is called classful.
I think depending on the class the price also varied, because in class A a company would get many private IPs.

With the growth of the internet, IANA understood they would quickly have no more available networks in this way. Nor would there be enough routes.
They had to figure a way out, to further subidivde,
hence CIDR was created. The solution forthe problem: not enough addreses
was network prefix. This is called classless.

If you want the addressess
195.0.0.0 to 195.255.255.255
you could use a network prefix of 8 bits. The first 8 bits (first byte) is always the same.

If you want
195.229.8.0 to 195.229.11.255
it wouldnt' be possible for a classful. The company would need to buy many addresses before classles was possible. However, with network prefix this is possible.
The first 16 bits and next 6 bits are always the same

195.229 8 and 11
first 16 bits 00001000
(= 2 byte) 00001011

With VLSM the idea is that you can further divide (CIDR) the network.
You can create for example

201.45.222.1100 XXXX
201.45.222.1110 XXXX
201.45.222.1111 XXXX

which was not possible with classful. Note that some routing protocols dont support classless. Classful protocols are RIPv1 and IGRP. Protocols that support VLSM are classless protocols: RIPv2, EIGRP and I think OSPFv2


VSLM works very much like CIDR. The concept of VLSM and CIDR is very very much alike, the difference is VLSM deals with subnets of a single network in a private organisation. CIDR takes the concept to the whole internet.