Route Selection Through The Routing Table - 3Com 7700 Configuration Manual

60 C 5: R
HAPTER OUTING
Route Selection through
the Routing Table
P O
ROTOCOL PERATION
Figure 1 About Hops
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Networks can have different sizes so the segment lengths connected between two
different pairs of routers are also different.
If a router in a network is regarded as a node and a route segment in the Internet
is regarded as a link, message routing in the Internet works in a similar way as the
message routing in a conventional network. Routing a message through the
shortest route may not always be the optimal route. For example, routing through
three LAN route segments may be much faster than a route through two WAN
route segments.
Tor the router, a routing table is the key to forwarding packets. Each router saves a
routing table in its memory, and each entry of this table specifies the physical port
of the router through which a packet is sent to a subnet or a host. Therefore, the
packet can reach the next router over a particular path or reach a destination host
through a directly connected network.
A routing table has the following key entries:
A destination address — Identifies the destination IP address or the destination
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network of the IP packet, which is 32 bits in length.
A network mask — Is made up of several consecutive 1s, which can be
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expressed either in the dotted decimal format or by the number of the
consecutive 1s in the mask. Combined with the destination address, the
network mask identifies the network address of the destination host or router.
With the destination address and the network mask, you have the address of
the network segment where the destination host or router is located. For
example, if the destination address is 129.102.8.10, the address of the
network where the host or the router with the mask 255.255.0.0 is located is
129.102.0.0.
The output interface — Indicates an interface through which an IP packet
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should be forwarded.
The next hop address — Indicates the next router that an IP packet will pass
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through.
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