D-Link DES-3326SR User Manual page 89

DES-3326SR Layer 3 Fast Ethernet Switch User's Guide
Figure 5-11. IP Packet Header
The flags and fragment offset are used to keep track of packets
that must be divided among several smaller packets to cross
networks for which they are too large.
The Time-to-Live (TTL) is the number of gateways the packet is
allowed to cross between the source and destination. This
number is decremented by one when the packet crosses a
gateway and when the TTL reaches zero, the packet is dropped.
This helps reduce network traffic if a loop develops.
Ethernet
Every active Ethernet device has its own Ethernet address
(commonly called the MAC address) assigned to it by the
manufacturer. Ethernet uses 48 bit addresses.
The Ethernet header is 14 octets that include the source and
destination MAC address and a type code.
There is no relationship between the MAC address of a network
node and its IP address. There must be a database of Ethernet
addresses and their corresponding IP addresses.
Different protocol families can be in use on the same network.
The type code field allows each protocol family to have its own
entry.
A checksum is calculated and when the packet is received, the
checksum is recalculated. If the two checksums are different,
the packet is dropped.
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