The following table shows the network number and host ID arrangement for classes A, B and C.
Table 136 Classes of IP Addresses
IP ADDRESS | OCTET 1 | OCTET 2 | OCTET 3 | OCTET 4 |
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Class A | Network number | Host ID | Host ID | Host ID |
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Class B | Network number | Network number | Host ID | Host ID |
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Class C | Network number | Network number | Network number | Host ID |
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An IP address with host IDs of all zeros is the IP address of the network (192.168.1.0 for example). An IP address with host IDs of all ones is the broadcast address for that network (192.168.1.255 for example). Therefore, to determine the total number of hosts allowed in a network, deduct two as shown next:
•A class C address (1 host octet: 8 host bits) can have 28 – 2, or 254 hosts.
•A class B address (2 host octets: 16 host bits) can have 216 – 2, or 65534 hosts.
A class A address (3 host octets: 24 host bits) can have 224 – 2 hosts, or approximately 16 million hosts.
IP Address Classes and Network IDThe value of the first octet of an IP address determines the class of an address.
•Class A addresses have a 0 in the leftmost bit.
•Class B addresses have a 1 in the leftmost bit and a 0 in the next leftmost bit.
•Class C addresses start with 1 1 0 in the first three leftmost bits.
•Class D addresses begin with 1 1 1 0. Class D addresses are used for multicasting, which is used to send information to groups of computers.
•There is also a class E. It is reserved for future use.
The following table shows the allowed ranges for the first octet of each class. This range determines the number of subnets you can have in a network.
Table 137 Allowed IP Address Range By Class
CLASS | ALLOWED RANGE OF FIRST OCTET (BINARY) | ALLOWED RANGE OF FIRST | |
OCTET (DECIMAL) | |||
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Class A | 00000000 to 01111111 | 0 to 127 | |
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Class B | 10000000 to 10111111 | 128 to 191 | |
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Class C | 11000000 to 11011111 | 192 to 223 | |
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Class D | 11100000 to 11101111 | 224 to 239 | |
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Class E | 11110000 to 11111111 | 240 to 255 | |
(reserved) |
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302 | Appendix C IP Addresses and Subnetting |