Address ClassesThere are 5 different address classes. You can determine which class any IP
address is in by examining the first 4 bits of the IP address.
- Class A addresses begin with 0xxx, or 1 to 126
- Class B addresses begin with 10xx, or 128 to 191
- Class C addresses begin with 110x, or 192 to 223
- Class D addresses begin with 1110, or 224 to 239
- Class E addresses begin with 1111, or 240 to 254
Addresses beginning with 01111111, or 127 decimal, are
reserved for loopback and for internal testing on a local machine. Class D
addresses are reserved for multicasting. Class E addresses are reserved for
future use. They should not be used for host addresses.
Now we can see how the Class determines, by default, which part of the IP
address belongs to the network (N) and which part belongs to the Host/node
- Class A: NNNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH
- Class B: NNNNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH
- Class C: NNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH
In the example, 192.189.210.078 is a Class C address so by default the
Network part of the address (also known as the Network Address) is
defined by the first three octets (192.189.210.XXX) and the node part is
defined by the last one octets (XXX.XXX.XXX.078).
In order to specify the network address for a given IP address, the node section is set to all "0"s. In our example, 126.96.36.199 specifies the network address for 192.189.210.078. When the node section is set to all "1"s, it specifies a broadcast that is sent to all hosts on the network. 188.8.131.52 specifies the broadcast address.
Private SubnetsThere are three IP network addresses reserved for private networks. The addresses are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. They can be used by anyone setting up internal IP networks, such as an intranet. Internet routers never forward the private addresses over the public Internet.