48
Chapter 2
Protocol Attributes
These bit specifications not only defined the IP class, but also predefined
the shortest subnet mask for the address.
The assigned masks are depicted in Table 2.4, wherein the prefix for each
class varies from eight bits to 24 bits. You can see a prefix of eight bits with
the first bit set to 0 (2
7
), which allows for 128 Class A networks. The Class
B prefix of 16 bits, with the first bit set to 1 and the second set to 0 (2
14
),
allows for 16,384 Class B networks. Finally, the Class C prefix of 24 bits,
with three bits being used for class definition (2
21
), allows for 2,097,152
Class C networks. As you can see, the available network numbers, using the
classfull scheme, is finite. Although 2,097,152 networks seems like a great
number of networks, when you look at it within a global frame of reference,
you can see that they can eventually run out.
Because this is just a review, we leave IP addressing at this point and move
on to some of the other protocols that are used within the IP suite.
T A B L E 2 . 3
RFC 791 IP Class Assignments
Address Class
Bit Specification
A
0
B
10
C
110
T A B L E 2 . 4
Classfull IP Subnet Mask Assignments
Address Class
Subnet Mask
A
255.0.0.0
B
255.255.0.0
C
255.255.255.0
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