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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Figure 5-14. 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.
Figure 5-15. Ethernet Packet Header
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