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I have just graduated to 3th semister of CCNA and the 1st topic is on VARIABLE LENGHT SUBNETMASKS,so i was just wondering if there is any one in the forum who can help me on how to learn to calculate VLSM's
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Re: ASSISTANCE NEEDED ON CALCULATING VLSM 13 years 8 months ago #9311

  • jwj
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I would suggest reading the guide to subnetting here. Doesn't CCNA Academy teach subnetting? If you need something in particular explained more in-depth/in a different way, it would help us help you out. It start here:

Re: ASSISTANCE NEEDED ON CALCULATING VLSM 13 years 8 months ago #9315

Perhaps you can be more specific as to what part of the subnetting process you don't quite understand? Sometimes tutorials don't answer every question you might have about the process. Feel free to post them here :-)
"He who breaks something to find out what it is, has left the path of wisdom."

Gandalf the Grey

Re: ASSISTANCE NEEDED ON CALCULATING VLSM 13 years 8 months ago #9371

ive never explained this before, but ill give it a shot.

Just to give a quick summary. Its really just subnetting a subnet.

if you have subnetwork range of -
you can subnet this range of IPs even further. Mostly commonly for point to point links. e.g. A Router connecting to another Router. - - - -

you now have created 4 more networks from the original subnetwork. Which allows us to use Our IP address in a more efficient manner. Even though this diagram is pretty weak, i hope you can see where this comes into play.
Network1 can use the range -
Network2 can use the range -

Network1 Network2
| |
| R1 ? R2 |
| |

(okay this digram wont line up right :( i tried)

we still need the connection between the routers to be a network as well, so instead of using another one of the big address ranges, we use one of the ones we used VLSM on.

R1-R2 connection = -
(you can see this leaves us with 2 IPs for the router interfaces and 2 for the Network address & Broadcast Address)

The only other thing to be aware of is routing protocols. You need a classless routing protocol. This means that when communcating with other routers its sends the Subnet mask info with the IP address info. For VLSM to be functional on a network, you need to be using a classless routing protocol. Classless Protocols: RIPv2, OSPF

if i missed anything guys, let me know :)
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