Subnetting

Close.

First of all, you need to look, not only at the number of subnets, but the number of users that will be on that subnets (this include networked devices, such as networked printers)

With 255.255.255.0 (/24), you are not subnetted, unless it is a class A or B address. Now this does not take into account that you can be given a class A or class B address from your ISP, that is already subnetted. It is common to give you an address of 66.128.15.22 and have a subnet 255.255.255.0. This means that the ISP has subnetted his address. You can then subnet it yourself (/27, in this case - 255.255.255.224).

If you use /27 you will have 6 (not 5) subnets (borrowing 2 only gives you 2, so that is out)

2^3=8-2=6

Number of users is

2^(8-3)=2^5=32-2=30.

If you have more than 30 people (and devices) in any of the departments, you have to rethink the network setup.

Wow,

Nice job indezblue and tfs! Will I ever get to the point where I can quote those formulae off of the top of my head :shock: ? Or did you guys look 'em up? :roll: that's why I actually cheat :oops: and use a subnet calculator, although I know it won't help me learn it, I almost never have time to hit reference books to figure it out...<sigh>

About the first address in a subnet, you can't assign that due to the fact of when you want to add a network route you point to that first address. If you assign it to a host, you will direct all traffic for that net to that particular host. Giving assumptions, of course.

I wish I knew other pros in my personal/social circle in order to have some face to face on subjects like this. My substitute? I explain stuff to my wife of 24 years and believe it or not, over time she has grasped some pretty tough concepts enough to at least tell her of my exploits. She ACTUALLY sees the irony/chagrin of issues and monologues have turned to dialogues.....Hahaaaa.

subnet calculator !! tch tch.. thats a banned word around here.. actually two banned words

Frankly subnetting gets confusing only because there are many exceptional cases.. not to mention with the desperate drive to save IPv4 they've been mangling the original standards of whats allowed and whats not allowed.

Its a very important concept but you're right theres no need to do it manually each time. Take my case for example.. I don't do much network design, but I do occasionally need to calculate ranges and suchlike.. I know how to subnet in my head (pretty much a requirement for any Cisco cert), but now I've come to the point where I do it so rarely that when I do I just use a calculator.. also there are certain common masks that you recognise and you instantly know the ranges.

Despite what all macho networking gurus will try and tell you, you could even survive with a printed chart without never ever converting a number to binary..

But then you wont ever become one of us pure blooded elite types


I think I can safely say that Chris has written the best subnetting article on the net. I read just about every single one when I was starting out because I found it confusing. This one really cleared things up for me.

I agree.

Chris's article is the best - bar none. It has plenty of diagrams that are easy to follow.

Subnetting is pretty confusing, but after you get the basics and do a few examples yourself (100 or so) - you should have no problem.

Thanks for the info, it helped a lot. One other question, where I can find that Chris' article?

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Networking >> Protocols >> Internet Protocol - Subnetting

Its broken up into different sections so that you don't have to digest the whole thing in one sitting..

For best results we recommend you sit back with a latte, maybe a few french fries, and enjoy yourself. Grab a sheet of paper as well so you can write stuff down and try a few out.

If you want some of the sneakiest subnetting tricks going -- check out Todd Lammles CCNA study guides (that man should give me a commission on every book of his I've promoted)... I use all his dirty little tricks.. so people look at me and watch in awe as I subnet in my head, spewing number of hosts and valid ranges all along.... hehe if they knew how simple it was...