Most devices on a network should have an address to identify them and hence make them reachable. If you are sending data to a certain computer, how will this datagram reach its destination? Simply, this PC will have an address. Actually there are two kinds of addresses assigned for network devices: Physical Address (MAC Address) This address is a permenant one which is burnt-in the ROM of the network device ( Network Interface Card{NIC} in case of computers). It cannot be changed unless you replace the device itself . This address is composed of 48 bits which are expressed in the form of 12 Hexadecimal digits. For example: 00-30-a5-35-81-4c Once this address is burnt to a device , it cannot be used or assigned for another device. The left 6 hexadecimal digits are called the Organizational Unique Identifier as they identify the organization or the company that manufactured this address. The right 6 hexadecimal digits are called the Vendor Assigned as they identify a serial number in the vendor's line of production .This address has many names including physical, burnt-in, hardware and MAC address(Media Access Control). Logical Address ( IP Address) This address is a temporary address which can be changed if wanted . It is assigned to a device using a protocol named Internet Protocol or IP. The address is formed of 32 bits which are expressed in the form of decimal dotted notation to make it easier to work with. This notation is formed of 4 decimal numbers seperated by dots ( Each number being composed originally of 8 bits). For Example : 200.100.1.2 This Address is composed actually from two portions : a Network ID portion and a Host ID portion . The network id identifies the network to which the host belong , while the host portion identifies the host itself.