|  | 9.4 | LAN Switching
| 9.4.4 | Functions of a MAC address - the hub
In Figure , there are four PCs connected to a hub. When a hub receives an Ethernet frame on one port, it forwards (repeats) it out all other ports. For example, if Host A is sending an Ethernet frame to Host D, Host A encapsulates the data and upper layer headers into the Layer 2 frame field known as the “Data.” Normally, when data is being sent from one computer to another, several frames may be needed to transmit it all. In the Ethernet frame header, as shown in Figure , Host A is the Source MAC address and Host D is the Destination MAC address. Question: After Host A sends out the frame, which Hosts will see this frame, more specifically, where does the frame go? Answer : The answer is that all of the Hosts on this network, every host connected to Hub, will see at least part of this frame. Explanation: Once the frame is sent out from Host A to the hub, the hub forwards (repeats) the frame out all ports (except the port the frame came in on). Therefore, each Host connected to the hub begins to receive the frame. Each host copies the beginning of the frame, the MAC Destination Address, into the NIC card. If this Destination MAC address matches the MAC address of the NIC card, then the host copies in the rest of the frame. If the Destination MAC address does not match the NIC card address, then the host ignores the rest of the frame. If any other Host on the hub sends an Ethernet frame at the same time Host A is sending its frame as shown in Figure , a collision will occur because all four hosts are on the same collision domain.