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Each host on a TCP/IP network has a default gateway, an off-ramp for datagrams not destined for the local network. They’re going somewhere else, and the gateway’s job is to forward them to that destination if it knows where it is. Each gateway has a defined set of routing tables that tell the gateway the route to specific destinations.

Because gateways don’t know the location of every IP address, they have their own gateways that act just like any TCP/IP host. In the event that the first gateway doesn’t know the way to the destination, it forwards the datagram to its own gateway. This forwarding, or routing, continues until the datagram reaches its destination. The entire path to the destination is known as the route.

Datagrams intended for the same destination may actually take different routes to get there. Many variables determine the route. For example, overloaded gateways may not respond in a timely manner or may simply refuse

to route traffic, and so they time out. That time-out causes the sending gateway to seek an alternate route for the datagram.

Routes can be predefined and made static, and alternate routes can be predefined, providing a maximum probability that your datagrams travel via the shortest and fastest route.

The Application Protocols

he following 12 applications were built on top of the TCP/IP protocol suite and are available on most implementations.

Simple Network Management Protocol (SNMP)

SNMP allows network administrators to collect information about the network. It is a communications protocol for collecting information about devices on the network, including hubs, routers, and bridges. Each piece of information to be collected about a device is defined in a Management Information Base (MIB). SNMP uses UDP to send and receive messages on the network.

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