3/20/2023 0 Comments Fcs checksum calculatorIf these tests checks out ok you could say that the NICs (and layer two at the whole) are almost “done”. The NIC will also run a mathematical checksum calculation (called CRC32) of the content of the frame and compare to the checksum attached at the tail of the frame. If all of these address checks fails (and the NIC is not in promiscuous mode) the frame will be dropped. It will verify that the “Destination MAC” is the either the NICs own unicast address, or if not, check if it might be destinated to the general Ethernet Broadcast Address (FF-FF-FF-FF-FF-FF) or if not, verify if the destination address is a Ethernet Multicast Address that the NIC has been instructed to “listen” on. When a certain frame has reached the target MAX addess the NIC will procede with several checks. The third and final field is called “ Ethertype” and will be discussed below. MAC addresses are typically written in a hexadecimal representation, for example AB-DC-EF-12-34-56. Both these fields consumes 6 bytes (which is 48 bits, which is the size of an Ethernet MAC address). These fields are used for the Layer 2 equipment (switches) to deliver the frame to the correct NIC inside the LAN. The first two are “ Destination MAC Address” and “ Source MAC Address“. The Ethernet header contains three fields. (For information on the “space” between frames on a wire, called preamble and the interframe gap, please see this article. ![]() This checksum is typically verified at each switch port as the frame travels through the LAN and also at the target Network Interface Card (NIC).Īt the very beginning of each frame a 14 byte long “Ethernet header” is located. Some of the space is however used by Ethernet itself.Īt the end of each frame we will find a 4 byte checksum called FCS. This frame has a minimum size of 64 and a maximum size of 1518 bytes. How to understand the Ethertype field in Frame headerĪll data being sent on an Ethernet based network is encapsulated in a “frame”.
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