System information

Appendix 2
Network Primer
Introduction
This appendix is intended to aid novice network technicians understand and build a routed network.
This text will not be all-encompassing, but is intended to get you over the initial hurdle of subnetting
and routing an IPv4 network.
Background
For those who have been assigned a “Class C” network by their providers, but don't know where to go
from here, this text will provide that help. It is really simple once you understand.
First, I'd like to point out that us “old-timers” still use the terms Class A, Class B, and Class C when
referring to network blocks. Understand, however, that these Classes were made obsolete around 1996
when Classless Inter Domain Routing (CIDR) was standardized. This text will not discuss CIDR, but
will restrict itself to the subset of CIDR in use globally known as Variable Length Subnet Masking
(VLSM).
To understand VLSM, it is important to grasp where this concept comes from. So first we'll delve
lightly into IPv4 and just how an IP address is composed, and from there we'll be able to see how
VLSM works.
After understanding how VLSM works, we'll look at how to easily subnet your “Class C” address
block. Basically, if you can multiply and divide by 4, 8, 16, 32, 64, and 128 numbers from 1-255,
you've got it made.
IP
IP addresses are composed of 4 decimal digits separated by dots. This notation is called “dotted
decimal notation”. Each set of decimal numbers can range from 0 to 255. The astute student will note
that this equates in binary to the numbers from 00000000 to 11111111. Note that this is eight places in
binary. So each number is sometimes referred to as an “octet”.
Nothing difficult here. All we have to do is think of each number (0-255) as a binary octet.
If you translate each octet to its binary form from decimal, you remove the decimal dots between the
octets and translate the entire number into decimal, you will get a unique decimal number equivalent to
the IP address. While this is not a necessary exercise to accomplish, it will help you understand how