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A single gateway is usually defined as a default gateway, if
that gateway connects the local network to a backbone
network or to the Internet. This default gateway is also used
whenever no specific route is found for a packet, or when
there are several gateways on a network.
Local computers can use default gateways, but the
gateways themselves need a more complete routing table to
be able to forward packets correctly. A protocol is required
for the gateways to be able to communicate between
themselves and to keep their routing tables updated.
Packet Fragmentation and Reassembly
TCP/IP can be used with many different types of networks,
but not all network types can handle the same length
packets.
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.
162 VH-2402-L3 Management Guide
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