Someone else on your network is using your IP address.
Hardware address is a unique identifier of a network interface card (NIC, a.k.a. Ethernet card), the piece of electronics in a computer that the network cable is plugged into, usually using a jack on the back of the computer. This is built into most Macs, laptops and pinters. In many PCs, some Macs and laptops the NIC is an add-on card. The hardware address is used to identify the machine on the network so an IP address can be assigned to it by a server. Therefore, it is required for all IP requests.
FYI: hardware address is also called a MAC number which is not to be confused with a Mac