238 Chapter 5: Network Protocols
Many different Class A, B, and C networks exist. If connecting to the Internet without using a
form of Address Translating Gateway (such as the Cisco PIX), then your firm must use
registered, unique network numbers. To that end, the Network Information Center (NIC)
assigns networks so that uniqueness is achieved. Table 5-11 summarizes the possible network
numbers, the total number of each type, and the number of hosts in each Class A, B, and C
network.
*
Valid Network Numbers column shows actual network numbers. There are several reserved cases. For example,
network 0.0.0.0 (available for use as a broadcast address) and 127.0.0.0 (available for use as the loopback address)
are reserved. Networks 128.0.0.0, 191.255.0.0, 192.0.0.0, and 223.255.255.0 are also reserved.
Classifying a network as Class A, B, or C should become an instantaneous process before you
take the test. Memorize the ranges in the second column of Table 5-11. Also memorize the
number of octets in the network part of Class A, B, and C addresses, as shown in Table 5-9.
Masks and IP Address Formats
One common task that CCNAs run into is the interpretation of a network or subnet mask. This
mask is used for several purposes. One key purpose is to define the number of host bits in an
address. This mask also is used by computers when calculating the network or subnet number
of which that address is a member.
Table 5-10
Example Network Numbers, Decimal and Binary
Network Number
Binary Representation, with Host Part Bold
8.0.0.0
0000 1000 0000 0000 0000 0000 0000 0000
130.4.0.0
1000 0010 0000 0100 0000 0000 0000 0000
199.1.1.0
1100 0111 0000 0001 0000 0001 0000 0000
Table 5-11
List of All Possible Valid Network Numbers*
Class
First Octet
Range
Valid Network
Numbers
Total Number
of This Class of
Network
Number of
Hosts per
Network
A
1 to 126
1.0.0.0 to
126.0.0.0
2
7
minus two
special cases
2
24
minus two
special cases
B
128 to 191
128.1.0.0 to
191.254.0.0
2
14
minus two
special cases
2
16
minus two
special cases
C
192 to 223
192.0.1.0 to
223.255.254.0
2
21
minus two
special cases
2
8
minus two
special cases
ch05.fm Page 238 Monday, March 20, 2000 5:06 PM