156
Chapter 3
Internet Protocols
Since 128 is 1000000 in binary, there is only one bit for subnetting. Since
this bit can be either off or on, the two available subnets are 0 and 128. You
can determine the subnet value by looking at the decimal value of the fourth
octet. If the valid of the fourth octet is below 128, then the host is in the 0 sub-
net. If the fourth octet value is above 128, then the host is in the 128 subnet.
The following table shows you the two subnets, valid host ranges, and
broadcast addresses for the Class C 255.255.255.128 mask.
So, if you have an IP address of 192.168.10.5 using the 255.255.255.128
subnet mask, you know it's in the range of the 0 subnet and bit number 128
must be off. If you have an IP address of 192.168.10.189, then 128 must be
on, and the host is considered to be in the 128 subnet. You'll see this again
in a minute.
Okay, this is a point of much confusion. Cisco says it is okay to use this mask
in production, but then considers this invalid or illegal on the Cisco exams.
Cisco is saying, "do as we say, not as we do." Remember for the exam that
one subnet bit is considered wrong and will be considered invalid.
Subnetting in Your Head: Class C Addresses
I'm not lying--I promise! It really is possible to subnet in your head. Even
if you don't you believe me, I'll show you how. And it's not all that hard
either--take the following example:
192.168.10.33
= Node address
255.255.255.224
= Subnet mask
First, determine the subnet and broadcast address of the above IP address.
You can do this by answering question 3 of the big five questions. 256
- 224
= 32. 32 + 32 = 64. Woohooo! The address falls between the two subnets and
must be part of the 192.168.10.32 subnet. The next subnet is 64, so the
Subnet
0
128
First host
1
129
Last host
126
254
Broadcast
127
255
Copyright ©2002 SYBEX, Inc., Alameda, CA
www.sybex.com