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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
108 Switch Management and Operating Concepts
Gateways connect a network with one or more other networks.
Gateways can be a computer with two network interfaces or a
specialized device with multiple network interfaces. The device
is designed to forward packets from one network to another.
IP routing is based on the network address of the destination IP
address. Each computer has a table of network addresses. For
each network address, a corresponding gateway is listed. This
is the gateway to use to communicate with that network. The
gateway does not have to be directly connected to the remote
network, it simply needs to be the first place to go on the way
to the remote network.
Before a local computer sends a packet, it first determines
whether the destination address is on the local network. If it is,
the packet can be sent directly to the remote device. If it is not,
the local computer looks for the network address of the
destination and the corresponding gateway address. The
packet is then sent to the gateway leading to the remote
network. There is often only one gateway on a network.
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.