What are IP address classes?

IPv4 addresses are divided into five classes.  Each IP address class has a different maximum number of IPs that exist within it.

Class First Octet Range Default Subnet Mask Maximum Number of Hosts Per Network
A 1-127 255.0.0.0 16,777,214 Hosts (2^24-2)
B 128-191 255.255.0.0 65,534 Hosts (2^16-2)
C 192-223 255.255.255.0 254 Hosts (2^8-2)
D 224-239 N/A
E 240-255 N/A

Class A IPs start with 1-127.  Keep in mind, though, that 127 is reserved for localhost.  The remaining three octets are used for the host id.  Class B IPs start with 128-191 and use the first and second octets for the network id.  Class C begins with 192-223 and use the first three octets for the network id.  Class D and class E are not generally used for setting up networks.  Class D is Multicast and class E is experimental.

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