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cisco ccna salary.50 ESP51 AH88 EIGRP89 OSPFPacket Header Checksum field: Contains the checksum of the IP packet header. Whenever the router modifies the contents of the IP packet header (for example, by subtracting 1 from the TTL field value), it needs to recalculate the IP packet header checksum.Source IP Address field: The IP address of the host that sent the IP packet.Destination IP Address field: The IP address of the host that received the IP packet.1.4.2 IPv4 Fragmentation When the length of an IP packet exceeds the MTU value of the Layer 2 link it penetrates, it needs to be fragmented before it can be sent over that link. In practice, the maximum length of the net Layer 2 protocol load that encapsulates an IP packet depends on the physical medium of the Layer 2 link and certain other factors. For example, as mentioned earlier, the maximum length of the net load of a normal Ethernet frame that an Ethernet link can carry (also known as the maximum transmission unit [MTU]) is 1500 bytes. So what happens if a host sends out an IP packet that is longer than 1500 bytes? On an Ethernet link, packets need to be transmitted in pieces. Figure 1-45 shows an example of IPv4 packet fragmentation. Host A sends a 2000-byte IP data I bananamixture packet (including a 20-byte IP packet header). Before it is sent over the Ethernet link, the IP packet needs to be split into two slices.One slice is 1500 bytes and the other slice is 520 bytes (500 bytes for the rest of the IP net load, plus a new 20-word IP packet header). After all the pieces of the IP packet have been collected, the receiving host reassembles them into the original IP packet. The receiving host can identify two (or more) slices of the same IP packet because they have the same value for the packet ID field in the IP packet header. The marker field in the IP packet header has a bit named More Fragments (MF), which indicates whether the fragment is the last fragment of the IP packet. An MF position of 0 means that the slice is the last slice of the IP packet. The Slice Offset field is used to indicate the exact location where the slice starts from the original unsliced IP packet. 20 1980