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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 109
When IP is transmitting large files, large packets are much
more efficient than small ones. It is preferable to use the
largest possible packet size, but still be able to cross networks
that require smaller packets.
To do this, IP can ‘negotiate’ packet size between the local and
remote ends of a connection. When an IP connection is first
made, the IPs at both ends of the connection state the largest
packet they can handle. The smaller of the two is selected.
When a IP connection crosses multiple networks, it is possible
that one of the intermediate networks has a smaller packet size
limit than the local or remote network. IP is not able to
determine the maximum packet size across all of the networks
that may make up the route for a connection. IP has, therefore,
a method to divide packets into multiple, smaller packets to
cross such networks. This division of large packets into
smaller packets is referred to as fragmentation.
A field in the TCP header indicates that a packet has been
fragmented, and other information aids in the reassembly of
the packets into the original data.
Gateways that connect networks of different packet size limits
split the large packets into smaller ones and forward the
smaller packets on their attached networks.
ARP
The Address Resolution Protocol (ARP) determines the MAC
address and IP address correspondence for a network device.
A local computer will maintain an ARP cache which is a table of
MAC addresses and the corresponding IP addresses. Before a
connection with another computer is made, the local computer
first checks its ARP cache to determine whether the remote