DH Guide: Subnet Masking

***Zardon*** · Sep 28, 2004, 12:22 PM

Forum member Greg "Gutterpunk" Suhr has put together a rather indepth guide detailing subnet masking.

When hearing Subnet Masking, most people just cringe because of the horror stories that are shared throughout networking. In actuality it is fairly simple. The main things you need to understand are:

Binary to decimal conversions, and decimal to binary
Understanding on how to separate network bits to host bits
A brief understanding of the different classifications of IP addresses

In this guide I will cover each of these aspects starting with converting binary to decimal and visa versa. In subnet masking you work with a series of 4 bytes each byte separated by a decimal. This makes up your IP Address. Remember 1 bit = 8 bytes. So each Address contains 8 bytes or 4 bits.

read here

Gardinen · Sep 28, 2004, 12:47 PM

On Cisco 2600 series routers, you can you the [subnet zero] command in order to use the first subnet.

Vidmaster23 · Sep 28, 2004, 01:44 PM

Very good guide.

Some good additions to this guide would be Hexidecimal to binary and Binary to hexidecimal.

No_Style · Sep 28, 2004, 02:22 PM

Nice. Very nice!

I'm taking CISCO now, and a lot of people will benefit from this in my class. I've referred one to your guide. LOL If you could get VLSM/CIDR subnet masking in that would be even better!

Oh, and the 2 subnets not being used aren't really illegal when doing regular subnetting, it's just that one is for broadcasting all the networks and the other is used to ID the network, I believe.

VLSM/CIDR allows you to use those "illegal" subnets. VLSM is basically it's subnetting a subnet. Funky stuff. While CIDR is subnetting a subnet, but also borrowing more than the alotted 8 bits.

Manual subnetting isn't hard, but if you make a mistake it can be brutal which is why subnet calculators are around.

germanjulian · Sep 28, 2004, 03:05 PM

nice read. thank god this semester I am learning TCP/IP fully!!!! fully i mean fully...

Falstaff · Sep 28, 2004, 03:34 PM

looks like gold medal work to me. but then he always paid attention in class. LOL

drakesteakn · Sep 28, 2004, 05:00 PM

he doesn't pay attention in class. he plays vid games. he just learns quick. nice work greg. i really enjoyed reading it

GutterPunk · Sep 28, 2004, 05:14 PM

they are illegal because you cannot use them for an actual network

and thanks for the comments i worked pretty hard on this puppy

Axel Foley · Sep 28, 2004, 05:21 PM

Quote:

In this guide I will cover each of these aspects starting with converting binary to decimal and visa versa. In subnet masking you work with a series of 4 bytes each byte separated by a decimal. This makes up your IP Address. Remember 1 bit = 8 bytes. So each Address contains 8 bytes or 4 bits.

Nice job, just a couple of things:

1. The beginning of the article, which I quoted above, confuses bits and bytes (1 byte=8 bits).

2. An IP address is 4 bytes (32 bits).

A beginner would get confused by reading the rest of the article if these two simple things aren't correctly explained.

Anyway good job Greg, keep it up.

GutterPunk · Sep 28, 2004, 05:35 PM

ooops lol.... 4 bytes or 32 bits... i didnt really get them confused just messed up lol thx for pointing that out

BWX · Sep 28, 2004, 08:16 PM

Wow, nice guide/ info.

...will have to paste it into a text doc and study it, lots of info there.

kbleft · Sep 28, 2004, 08:32 PM

Also, the conversion between binary and decimal is not explained correctly at all. When discussing conversion from binary to decimal, the article has a table on the first page where the values of the binary digits are converted to decimal numbers 0 through 7 and then those numbers are squared. For example, the first line of the table states 1*7^2=128. This explanation is totally wrong.

Each binary digit represents a power of two. The digits correspond to the following powers of two from left to right: 7 6 5 4 3 2 1 0. Thus, on the lefthand side, the digit represents a possible increment of 2^7 or 128. Likewise, the second digit from the left represents a possible increment of 2^6 or 64, and so on. The digit on the right represents a possible increment of 2^0 or 1. The values of binary digits can be either 0 or 1. Accordingly, for the first digit on the lefthand side, a value of 1 represents an increment of 128, while a value of 0 represents no increment of 128. Thus, the proper expression for the decimal value of the Jth digit is either (0*2^J) or (1*2^J). It is never, ever, 1*7^2.

No_Style · Sep 28, 2004, 08:35 PM

Quote:

Originally Posted by GutterPunk

they are illegal because you cannot use them for an actual network

and thanks for the comments i worked pretty hard on this puppy

I would have said "unusable" or "reserved" instead of "illegal" because it makes it sound like some kind of crime if you do.

You can use the first network on a VLSM/CIDR network though or when you enable "ip subnet zero" on a router. With IOS version 12 and up its enabled by default. What semester of CISCO are you at? As you learn more and more, you'll realize how much the Internet was based on a foundation of poor planning.

Again, good work on the whole run down of it

P.S - Here's a nice document for those who want a nice charts relating to subnetting from Cisco! It will compliment Greg's work really really nicely!

Falstaff · Sep 28, 2004, 10:00 PM

I know it sounds cheezy but I used the calculator in windows accessories to do the conversions.

GutterPunk · Sep 28, 2004, 10:51 PM

Quote:

Originally Posted by kbleft

Also, the conversion between binary and decimal is not explained correctly at all. When discussing conversion from binary to decimal, the article has a table on the first page where the values of the binary digits are converted to decimal numbers 0 through 7 and then those numbers are squared. For example, the first line of the table states 1*7^2=128. This explanation is totally wrong.

Each binary digit represents a power of two. The digits correspond to the following powers of two from left to right: 7 6 5 4 3 2 1 0. Thus, on the lefthand side, the digit represents a possible increment of 2^7 or 128. Likewise, the second digit from the left represents a possible increment of 2^6 or 64, and so on. The digit on the right represents a possible increment of 2^0 or 1. The values of binary digits can be either 0 or 1. Accordingly, for the first digit on the lefthand side, a value of 1 represents an increment of 128, while a value of 0 represents no increment of 128. Thus, the proper expression for the decimal value of the Jth digit is either (0*2^J) or (1*2^J). It is never, ever, 1*7^2.

Wow I mustve mixed it up good call, as you are right... i dunno what i was thinking good thing i showed both ways... but I was thinking i had someone try to teach it to me that way thanks will be fixed

Falstaff · Sep 29, 2004, 02:18 AM

You put so much work into that Greg, and you ignored your women, tsk tsk....
more for me, woo hoo

GutterPunk · Sep 29, 2004, 10:19 AM

I proof read it so many times cant believe i didnt catch those

tek · Sep 29, 2004, 10:42 AM

I'm currently in a Cisco class, and we learned about all this stuff (binary to decimal, etc) in Module 1 (chapter 1). Pretty simple stuff... just use the Windows Calculator... lol