Although TCP/IP subnetting is not
exclusive to Windows 2000 (even NetWare admins now have to deal with it), we
thought we would devote this and the next Win2K News features to this very
important topic. TCP/IP is the default protocol for Windows 2000 and must be
installed in order for you to install Active Directory. Having an understanding
of TCP/IP, especially TCP/IP subnetting, is critical to being able to
administer a Windows 2000 network. For example, Windows 2000 Sites are defined
in terms of TCP/IP subnets. If you don’t understand how subnetting works, you
may have a difficult time configuring your Windows 2000 site topology, among other things.
First the Basics -- Binary Arithmetic
If you understand how binary numbers work, you can skip this section and go to
the next. But, if you don’t, you need to spend a bit of time here to get a
basic understanding of binary numbering.
All numbering systems work the same way. The one we are most familiar with,
Base 10 (decimal), works the same way as Base 2 (Binary) or Base 16
(Hexadecimal) for that matter. Let’s take a random number, such as 1,234. We
know immediately what that number is. It is so obvious to us that it seems
trite to say that the number is the sum of one thousand plus two hundred plus
thirty plus four. However, we can express this sum in another, more interesting
way:
(1 * 10^3) + (2 * 10^2) + (3 * 10^1) + (4 * 10^0) = 1,234 (The “^” symbol
indicates “raised to the power of”, eg “10^3” means 10 raised to the power of
3, or 10 * 10 *10)
It should be pretty obvious why we can refer to this number system as Base 10.
We have 10 digits to work with (0 – 9).
Binary (Base 2) numbering is like decimal (Base 10) numbering except that we
have only 2 digits to work with – 0 and 1. Let’s take a typical binary number,
such as 1101. Like Base 10 numbers, we can express a binary number as a sum of
other numbers. In the case of the example (1101), we can express the number
thus:
(1 * 2^3) + (1 * 2^2) + (0 * 2^1) + (1 * 2 ^ 0) = 8 + 4 + 0 + 1 = 13. (By the
way, any number raised to the power of “0” is “1”.)
As with decimal numbers, we can easily internalize a lot of the work we have to
do by looking at the positions of the digits in a particular number. Take a
longer binary number, such as the octet, 11111101. If you are familiar with
counting in binary you will know immediately that this number is 253. If you
are not familiar with counting in binary, don’t despair: it is not that hard.
Consider the table below.
But there is an even easier way to convert this number to decimal. We know that
the binary number 11111111 is 255 in decimal (you just have to memorize this).
When we look at a number like 11111101, all we have to do is subtract 2 (in
this case) from 255 to arrive at our decimal conversion, 253.
Although a TCP/IP address is 32 digits long, the address is broken up into 4
equal groups of 8 digits (4 groups of single bytes) to make it easier to work
with. So, here is a tip for working with binary numbers in TCP/IP addresses:
draw out the first 8 positions as in the table above. If you use that table,
you will be able to convert any decimal TCP/IP address to binary and vice
versa. One final tip: practice converting binary to decimal and vice versa;
check your results in the Windows calculator in scientific mode.
Calculating Subnet Masks
What are subnet masks and why do we need them? To answer these two questions,
we have to spend a little time discussing some basics about TCP/IP. A TCP/IP
address, such as 172.16.8.1, is composed of at least two parts: a section that
denotes the network portion of the address and a section that denotes the host
portion of the address. As an analogy, you can think of a postal address: one
part of the address refers to a street; another part refers to a particular
house. In order for the address to work properly, both parts have to be unique.
How does your computer “know” which portion or the TCP/IP address represents
the network and what portion represents the hosts? It looks at the subnet mask
for the address. The subnet mask distinguishes the network from the host
portion of the address (e.g., 172.16.8.1/16). Any bits in the address that are
“masked” by the subnet mask will represent the network portion (172.16.) of the
address; the remaining bits represent the host portion (.8.1) of the address.
When people look at a TCP/IP address, they often...
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