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IP "Classful" Addressing Network and Host Identification and Address Ranges
(Page 2 of 3)
Determining Address Class From the First Octet Bit Pattern
As humans, of course, we generally
work with addresses in dotted decimal notation and not in binary, but
it's pretty easy to see the ranges that correspond to the classes. For
example, consider class B. The first two bits of the first octet are
10. The remaining bits can be any combination of ones and
zeroes. This is normally represented as 10xx xxxx (shown
as two groups of four for readability.) Thus, the binary range for the
first octet can be from 1000
0000 to 1011
1111. This is 128 to 191 in decimal. So, in the classful
scheme, any IP address whose first octet is from 128 to 191 (inclusive)
is a class B address.
In Table 44
I have shown the bit patterns of each of the five classes, and the way
that the first octet ranges can be calculated. In the first column is
the format for the first octet of the IP address, where the xs
can be either a zero or a one. Then I show the lowest and highest value
for each class in binary (the fixed few bits are highlighted
so you can see that they do not change while the others do.) I then
also show the corresponding range for the first octet in decimal.
Table 44: IP Address Class Bit Patterns, First-Octet Ranges and Address Ranges
IP Address
Class
|
First
Octet of IP Address
|
Lowest
Value of First Octet (binary)
|
Highest
Value of First Octet (binary)
|
Range
of First Octet Values (decimal)
|
Octets
in Network ID / Host ID
|
Theoretical
IP Address Range
|
Class A
|
0xxx
xxxx
|
0000
0001
|
0111
1110
|
1 to 126
|
1 / 3
|
1.0.0.0
to 126.255.255.255
|
Class
B
|
10xx
xxxx
|
1000
0000
|
1011
1111
|
128
to 191
|
2
/ 2
|
128.0.0.0
to 191.255.255.255
|
Class C
|
110x
xxxx
|
1100
0000
|
1101
1111
|
192 to 223
|
3 / 1
|
192.0.0.0
to 223.255.255.255
|
Class
D
|
1110
xxxx
|
1110
0000
|
1110
1111
|
224
to 239
|
|
224.0.0.0
to 239.255.255.255
|
Class E
|
1111
xxxx
|
1111
0000
|
1111
1111
|
240 to 255
|
|
240.0.0.0 to
255.255.255.255
|
Key Concept: In the classful IP addressing scheme, the class of an IP address is identified by looking at the first one, two, three or four bits of the address. This can be done both by humans working with these addresses and routers making routing decisions. The use of these bit patterns means that IP addresses in different classes fall into particular address ranges that allow an addresss class to be determined by looking at the first byte of its dotted-decimal address. |
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Version 3.0 - Version Date: September 20, 2005
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