IPv4 Addressing

Class A

Devices whose first octet of their IP address is a value of 1-126. There are 126 available Class A networks and 16,777,214 addressable hosts per network. All IP addresses for devices on a Class A segment share the same first octet.

Class B

Devices whose first octet of their IP address is a value of 128-191. There are >16,000 available Class B networks and 65,534 addressable hosts per network. All IP addresses for devices on a class B network share the same first two octets.

Class C

Devices whose first octet of their IP address is a value of 192-223. There are 2,000,000 available Class C networks and 254 addressable hosts per network. Devices whose first octet of their IP address is a value of 128-191. There are >16,000 available Class B networks and 65,534 addressable hosts per network. All IP addresses for devices on a class C network share the same first three octets.

Subnet Mask

A special 32-bit number that, when combined with a device's IP address, informs the rest of the network about the segment or network to which the device is attached. The default subnet mask values are assigned according to network class: Class A (1-126) devices are assigned a default subnet mask of 255.0.0.0. Class B (128-191) devices are assigned a default subnet mask of 255.255.0.0 Class C (192-223) devices are assigned a default subnet mask of 255.255.255.0.

Class E

IETF (Internet Engineering Task Force) reserves Class E addresses, which begin with an octet whose value is between 240 and 254, for experimental use.

127.0.0.1

IP address 127.0.0.1 is called a loopback address. Attempting to contact this IP number—in other words, attempting to contact your own machine—is known as a loopback test. (In fact, when you transmit to any IP address beginning with the 127 octet, you are communicating with your own machine.) A loopback test can prove useful when troubleshooting problems with a workstation's TCP/IP communications. If you receive a positive response from a loopback test, you know that the TCP/IP core protocols are installed and in use on your workstation.

Class D

IP stressed that begin with an octet whose value is between 224 and 239. Reserved for multicasting.

IPv4 Addressing

Logical addresses that can be manually or automatically assigned and must follow rules set by the protocol standards. In the TCP/IP protocol suite, IP is the protocol responsible for logical addressing. For this reason, addresses on TCP/IP-based networks are often called IP addresses. IP addresses are assigned and used according to very specific parameters. Am UP address is unique 32-bit number, divided into four octets, or sets of 8 bits, that are separated by periods. Because 8 bits equals 1 byte, each octet is a byte, and an IP address is thus composed of 4 bytes.

Network ID

The first octet of an IP address, which indicates the class to which a node belongs.

255

When used in the host portion of an address, indicates a broadcast transmission. Sending a message to the address 147.82.255.255, for example, sends a message to all devices connected to the 147.82.0.0 network.