D-Link DGS-3308FG Product Manual - Page 51

8-port Gigabit Ethernet Switch User's Guide Figure 5-12. IP Packet Header Flags and Fragment Offset are used when a packet must be divided into smaller pieces by a network device. This is sometimes necessary for a packet to cross a network that can not process large packet sizes. Time-to-Live (TTL) is the maximum number of gateways a packet can pass through. This number is decremented each time a packet is forwarded through a gateway. When the TTL reaches zero, the packet is dropped. Ethernet Ethernet uses its own headers and addresses. Each Ethernet device or NIC card has a 48 bit Media Access Control (MAC) address assigned to it by the manufacturer. An Ethernet header is a 14 octets and includes the source and destination MAC addresses and a type code. For a computer or network device to have an IP address, a database must exist somewhere on the network to keep track of which MAC address corresponds to which IP address. The type code is used to specify one of several protocol families that may be in use on the network. A checksum is calculated an put at the end of the packet. A receiving device recalculates the checksum and if the two numbers are different, the packet is dropped. Figure 5-13. Ethernet Packet Header When the packet is received, these headers are removed. The Ethernet NIC removes the Ethernet header and checks the checksum. It looks at the type code. Since the type code is assigned to IP, the Ethernet device driver passes the packet to IP. 41