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How do you determine the network class of an IP address? For example, I have the IP address 127.0.0.1

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    IPv4 address classes have been deprecated 20 years ago. Since then CIDR (classless inter-domain routing) is used. Classes don't exist anymore. – Sander Steffann May 22 '14 at 4:42
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Network classes are deprecated. As a attempt to cope with IP address exhaustion, the concept of network classes has been dropped in 1992. Before that, for a company with a need for 300 IP addresses, a class C network with 255 addresses would not suffice and it therefore received a class B address space with 65535 addresses, thus wasting 65535-300=65235 addresses.

See the Wikipedia article on network classes for further info.

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Class Ranges: First octet
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Class A     1 - 127 

Class B   128 - 191

Class C   192 - 223
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Though 127.0.0.1 is class A, it's reserved for loopback/testing.

Source

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The following are the classes of IP addresses.

Class A The first octet denotes the network address, and the last three octets are the host portion.Any IP address whose first octet is between 1 and 126 is a Class A address. Note that 0 is reserved as a part of the default address, and 127 is reserved for internal loopback testing.

Class B The first two octets denote the network address, and the last two octets are the host portion.Any address whose first octet is in the range 128 to 191 is a Class B address.

Class C The first three octets denote the network address, and the last octet is the host portion. The first octet range of 192 to 223 is a Class C address.

Class D Used for multicast. Multicast IP addresses have their first octets in the range 224 to 239.

Class E Reserved for future use and includes the range of addresses with a first octet from 240 to 255

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