Definition - What Is an IP Address?

IP Address (Internet Protocol address) is as your telephone number. It uniquely identifies every host on a network. Just as your mailing address uniquely identifies your home, an IP address uniquely identifies a host.
Every host on a TCP/IP network needs to have a unique address, similar to you needing a unique address for your house. With this unique address, it is possible to send data from host to host. Every packet contains addressing information in the header, and the IP address in the header is used to route packets. If several people on your street had the same address, the post office would have a difficult time sorting mail. For a similar reason, IP addresses are unique on each network.

What is IP addressing?

IP addressing is simply configuring each TCP/IP host with a valid IP address. For access to the Internet, a host must have an IP address that identifies not only the host address (like a house number) but also identifies the network address (like a street number). An administrator needs to be aware of proper addressing techniques so that the hosts on the network will function correctly. TCP/IP addresses are based on 32-bit addresses.

IP Address Classes

IPv4 addresses are divided into five IP classes:

  • IP address class A
  • IP address class B
  • IP address class C
  • IP address class D
  • IP address class E

All IPv4 addresses are placed in a particular class based on the decimal values of their first octets. In the first octet, an IP address can start with a decimal value between 1 and 255.

  • IP class A addresses have first octets with a decimal number from 1 to 127. Example: 27.x.y.z 102.x.y.z
  • IP class B addresses have first octets with a decimal number from 128 to 191.Example: 128.x.y.z 151.x.y.z
  • IP class C addresses have first octets with a decimal number from 192 to 223.Example: 192.x.y.z 223.x.y.z
  • IP class D addresses have decimal values from 224 to 239 in the first octet, and the 4 leftmost bits are 1110.Example: 224.x.y.z 239.x.y.z
  • The last IP address class of addresses is IP class E. IP class E addresses range from 240 to 255 in the first octet, and the 4 leftmost bits are 1111. Example: 240.x.y.z 255.x.y.z

The system of IP class addresses has been set up to help ensure assignment of unique IP addresses. DHCP discover, DHCP offer, DHCP request, and DHCP acknowledgment are are four steps in getting an IP address from a DHCP server.