Classful Routing

Classful Routing
What is classful routing? Classful routing is used to route packets based upon the default major
network boundary, derived from the class of the IP address. In order to fully understand this
concept, let’s review the defaults for the different classes of IP addresses.
Class A networks reserve the first octet for the network portion of the IP address, and the
remaining three octets are available for host addressing. The value of the first octet of a Class A
network will always be between 1 and 127, inclusive. There are a total of 126 Class A networks;
127 is reserved for diagnostic testing and thus cannot be used. There are various other reserved
Class A networks, such as the 10 network, but the majority are usable and already allocated.
There are 16,777,214 unsubnetted hosts available per Class A network.
Class B networks reserve the first and second octets for the network portion of the IP address,
and the remaining two octets are available for host addressing. The value of the first octet of
a Class B network will always be between 128 and 191, inclusive. There are a total of 16,384
Class B networks with 65,534 hosts per network.
Class C networks reserve the first, second, and third octets for the network portion of the
IP address and the remaining octet for host addressing. The value of the first octet of a Class C
network will always be between 192 and 223, inclusive. There are a total of 2,097,152 available
Class C networks with 254 hosts per network.
Class D IP addresses have no network/host structure, as they are used solely for multicasting
and, like broadcast addresses, multicast addresses can be only destination addresses, never
source addresses. As a result, there is no need or way to split Class D addresses up into smaller
subnets, because no device will ever be configured with a Class D address as its interface
address. Furthermore, there is no subnet mask associated with Class D addresses. The value of
the first octet of a Class D address will always be between 224 and 239, inclusive. There are theoretically
268,435,456 Class D addresses, which are not split into networks and hosts.
Class E networks are regarded as experimental, and, like Class D addresses, they have no
network/host structure nor will they ever be assigned to a device’s interface. The value of the
first octet of a Class E address is always between 240 and 255, inclusive.
It’s not necessary to convert an IP address in binary form to decimal in order to determine its
class. When faced with such a task, the quickest way to determine the class of an IP address in
binary form is to label the first four bits A, B, C, and D, after the classes of addresses. Wherever
Metric Cost Cost Composite
Hop count limit Unlimited 1024 100 by default
Support for size of network Very large Very large Large
TABLE 1 . 2 Link-State Comparisons (continued)
Characteristic OSPF IS-IS EIGRP
18 Chapter 1  Routing Principles
the first 0 falls, that is the class of address you are dealing with. If all four bits are 1s, the class of
address is E. For example, a first octet of 10101010 would represent a Class B address, due to the
first 0 being in the B position. 10101010 converts to decimal 170, so you can see that the trick
worked in this case. Trust me, it always does.
Classful routing therefore bases all of its routing decisions upon the default major network
boundary derived from each of the first three classes of IP address. The major drawback to the
use of classful addressing and routing is that a tremendous number of IP addresses can be
wasted. We will explain this in more detail in Chapter 3.
The routing protocols covered in this book that are considered classful routing protocols are
as follows:
 RIPv1
 IGRP
With all of this in mind, let’s take a look at what is known as classless routing.