Although RRAS
provides multiprotocol routing capabilities, this discussion will focus on TCP/IP since
that is by far the most commonly used protocol on a Windows network.
Packets used in TCP/IP communication have source and destination addresses.
A subnet mask is applied to each address to determine which part of the IP address is the
network address and which part refers to the host. When a packet is being sent to its destination
by a host, it first determines whether the destination address is on the same subnet.
355 Chapter 11: Routing and Remote Access
If it is on the same subnet, the packet is simply sent out over the physical medium for
the destination host to pick up. If it determines that the destination is part of a different
network, it sends this packet to a router either defined by its routing table or, if no match
is found, to its default gateway.
It is the router??™s responsibility to examine packets being sent out to a different network
to determine where they should be sent off to next. The router intelligently determines
to which of its known interfaces it will send the packet to reach its final destination.
This is dictated by routing tables in the router that define rules that govern how packets
should be delivered, based on destination addresses. Some routers are configured with
redundant links to the same destination. For example, the router might be connected to
another network via a T1 line going to one Internet Service Provider (ISP) and another
T1 going to a completely different ISP.
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