Struggling with VLSM

bugs

Hiya
I'm currently working on my subnetting skills and have spent the last few days ensuring I have it mastered, which I seem to have apart from VLSM which I'm really struggling with. I have watched videos and read countless tutorials but it seems I can't work it out without the use of a cheetsheet. I would much prefer to not be reliant on memorizing tables.
Does anyone know of a good basic tutorial for it?

Specifically its questions such as 'You have been given the 172.16.0.0 network. Your boss has tasked you with creating 250 subnets that allow for up to 254 hosts each. What subnet mask would you use to achieve this goal?' which i`m sdtruggling with and where to start answering them

Thanks

p@r0tuXus

Memorizing tables is good for making the process of calculation faster. If you need to work on the principle concept, then it's easier to imagine those Russian nesting dolls. One fits inside another inside another. IP address space broken down for VLSM works in a similar way. You'll always use the biggest address space first. Let's say .128, then you add on the next biggest address space, perhaps a .64, which puts you at .192 for the beginning space of another address block. Add on another, smaller, address block for use... maybe a .32, and a .16, then a .8 (6 hosts, 1 network, 1 broadcast). Now you're sitting at .248 which is perfect for one last .8 or a couple of .4 address blocks (good for router to router).

I hope this helped. Essentially you always start with the biggest address block and work your way to the smaller ones to utilize all available addresses in the most efficient way. Also helps to make scalability easier if you have something come up later on down the road that requires use of address space.

rob42

SwedishTech has posted a link to a really good vid that covers VLSM [ http://www.techexams.net/forums/ccna-ccent/123883-do-classful-networks-use-broadcast.html ]. Have you seen it?

ImYourOnlyDJ

Most engineers use calculators and **** sheets in the real world. I personally do my subnetting mentally (if nessessary use a calculater to verify) or on my fingers so if it ever comes up I'm not useless without the calculator.

If you can watch the CBT Nuggets videos with Jeremy (can normally get a free one week trial if you don't have it). He explains this stuff in a very simple and easy to understand manner.

rob42

bugs wrote: »

'You have been given the 172.16.0.0 network. Your boss has tasked you with creating 250 subnets that allow for up to 254 hosts each. What subnet mask would you use to achieve this goal?' which i`m sdtruggling with and where to start answering them

Thanks

The only way I know of calculating the answer with pen and paper is...

2^8 = 256. Meaning that you'll need an extra 8 Network Bits to Address the 250 Subnets (in fact you could Address 256 Subnets). Mask of 255.255.255.0 (172.16.0.0/24) will achieve this.

That leaves 8 Bits for your Hosts which will Address 254 (256 - 2) Hosts.

Does that help?