3com 8807 User Guide
280
C
HAPTER
28: IP R
OUTING
P
ROTOCOL
O
VERVIEW
Figure 69 The concept of route segment
As the networks may have different sizes, the segment lengths connected
between two different pairs of routers are also different. The number of route
segments multiplies a weighted coefficient can serve as a weighted measurement
for the actual length of the signal transmission path.
between two different pairs of routers are also different. The number of route
segments multiplies a weighted coefficient can serve as a weighted measurement
for the actual length of the signal transmission path.
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. Message routed through the shortest
route may not always be the optimal route. For example, routing through three
high-speed LAN route segments may be much faster than that through two
low-speed WAN route segments.
is regarded as a link, message routing in the Internet works in a similar way as the
message routing in a conventional network. Message routed through the shortest
route may not always be the optimal route. For example, routing through three
high-speed LAN route segments may be much faster than that through two
low-speed WAN route segments.
Route Selection through
the Routing Table
The key for a router to forward packets is the routing table. Each router saves a
routing table in its memory, and each entry of this table specifies the physical port
of the router through which the packet is sent to a subnet or a host. Therefore, it
can reach the next router via a particular path or reach a destination host via a
directly connected network.
routing table in its memory, and each entry of this table specifies the physical port
of the router through which the packet is sent to a subnet or a host. Therefore, it
can reach the next router via a particular path or reach a destination host via a
directly connected network.
A routing table has the following key entries:
■
Destination address: It is used to identify the destination IP address or the
destination network of an IP packet.
destination network of an IP packet.
■
Network mask: Combined with the destination address, it is used to identify
the network address of the destination host or router. If the destination address
is ANDed with the network mask, you will get 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 will be 129.102.0.0. It
is made up of several consecutive "1"s, which can also be expressed in the
dotted decimal format.
the network address of the destination host or router. If the destination address
is ANDed with the network mask, you will get 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 will be 129.102.0.0. It
is made up of several consecutive "1"s, which can also be expressed in the
dotted decimal format.
■
Output interface: It indicates an interface through which an IP packet should
be forwarded.
be forwarded.
A
Route
segment
B
C
A
R
R
Route
segment
R
R
R
A