Now let's modify the small network in the
example in the previous section. The 192.168.1.x network is changed to
192.168.x.x and gateway B's address is changed to 192.168.10.1. All the
netmasks on the computers on the 192.168.x.x network are modified to 255.255.0.0
to accommodate the change, except machine S3 which keeps the netmask
255.255.255.0 and changes its address to 192.168.10.3. This effectively puts S3
on a different network than S2 and S1, it no longer believes it can talk
directly to them and must talk to gateway B to talk to them. It can't even talk
to gateway A anymore since it can't address it directly. Machines S1, S2, and A
are not on network 192.168.10.0, their addresses are 192.168.1.*. S1 and S2 can
talk to S3, but S3 will not be able to respond unless it utilizes gateway B.
Please be
aware, in the example in the previous section, that gateway A was aware of
gateway B. If it were not, no messages could have been transmitted from the
internet to the 192.168.2.0 network. In this example, gateway A knows nothing
about gateway B, and as far as it's concerned, the network 192,168.2.0 is part
of 192.168.0.0 and there is no gateway between them. Gateway B, does know about
gateway A and is using that gateway as its default gateway. Therefore if S1 and
S2 use gateway A for their default gateway, they will not be able to talk to
S4, 5, or 6 unless their routing table is modified. S1 and S2 will be able to
talk to S3, however, assuming S3 is using gateway B.
Destination
|
Gateway
|
Genmask
|
Flags Metric Ref Use Iface
|
||||
192.168.1.5
|
*
|
255.255.255.255
|
UH
|
0
|
0
|
0
|
eth0
|
192.168.0.0
|
*
|
255.255.0.0
|
U
|
0
|
0
|
0
|
eth0
|
127.0.0.0
|
*
|
255.0.0.0
|
U
|
0
|
0
|
0
|
lo
|
default
|
192.168.1.1
|
0.0.0.0
|
UG
|
0
|
0
|
0
|
eth0
|
Here it is modified to let
it use network 192.168.2.0.
Destination
|
Gateway
|
Genmask
|
Flags Metric Ref Use Iface
|
||||
192.168.1.5
|
*
|
255.255.255.255
|
UH
|
0
|
0
|
0
|
eth0
|
192.168.0.0
|
*
|
255.255.0.0
|
U
|
0
|
0
|
0
|
eth0
|
192.168.2.0
192.168.10.1
|
255.255.255.0
|
UG
|
0
|
0
|
0
|
eth0
|
|
192.168.2.0
|
*
|
255.255.255.0
|
U
|
0
|
0
|
0
|
eth0
|
127.0.0.0
|
*
|
255.0.0.0
|
U
|
0
|
0
|
0
|
lo
|
default
|
192.168.1.1
|
0.0.0.0
|
UG
|
0
|
0
|
0
|
eth0
|
It specifies the gateway
B, 192,168.10.1 to be used if the destination is 192.168.2.x.
The figure below shows an ethernet network with bus topology
excluding the hubs. It is a large Class A network with many subnetworks. The
machines labeled A through D are routers or potential routers and each have two
network interface cards(NIC). These machines may be called gateways since their
function is to be a gate to another location. Each card has a valid address on
its own network or subnetwork. The table below lists each gateway, and each NIC
address and associated network.
Gateway
|
eth0
|
eth0 network
|
eth1
|
eth1 network
|
A
|
10.0.0.1
|
10.x.x.x
|
164.25.74.131
|
Internet
|
B
|
10.0.0.2
|
10.x.x.x
|
10.1.0.1
|
10.1.x.x.
|
C
|
10.0.0.3
|
10.x.x.x
|
10.2.0.1
|
10.2.x.x.
|
D
|
10.0.0.4
|
10.x.x.x
|
10.3.0.1
|
10.3.x.x.
|
E
|
10.3.50.1
|
10.3.x.x
|
10.3.100.1
|
10.3.100.x.
|
F
|
10.1.0.2
|
10.1.x.x
|
10.1.20.1
|
10.1.20.x.
|
G
|
10.2.0.2
|
10.2.x.x
|
192.168.1.1
|
192.168.1.x.
|
H
|
10.3.100.2
|
10.3.100.x
|
10.3.150.1
|
10.3.150.x.
|
I
|
10.3.150.2
|
10.3.150.x
|
192.168.1.2
|
192.168.1.x.
|
In this figure, there are 9 gateways. which are labeled A
through I. There are multiple paths between several networks. The possible
paths between networks 10.1.100.x and 192.168.1.x can be through gateways E, D,
C, then G (E-D-C-G) or through gateways H-I. The path from 10.3.100.x ot
10.1.20.x can be E-D-B-F or H-I-G-C-B-F. Obviously there are ways to set the
routing paths up that may not be fully efficient. In this type of network, the
administrator must give careful thought to the setup of the routing tables in
their gateways. It would be easy to set up an infinite packet route loop in
this network where some packets may go in circles from router to router. Here's
how I would route for this network.
The below
table lists each network and their default router.
Network Default Router
10.3.100.x E
10.3.150.x H
192.168.1.x G
10.1.20.x F
10.1.x.x
|
B
|
10.2.x.x
|
C
|
10.3.x.x
|
D
|
10.x.x.x
|
A
|
The router, I, is not used
as a default router for any network.
The table below lists an
abbreviated route table for each gateway.
Router
|
Destination
|
Gateway
|
A
|
192.168.1.x
|
C
|
10.1.x.x
|
B
|
|
10.2.x.x
|
C
|
|
10.3.x.x
|
D
|
|
10.x.x.x
|
10.0.0.1
|
|
default
|
internet
|
|
B
|
10.1.20.x
|
F
|
10.1.x.x
|
10.1.0.1
|
|
default
|
A
|
|
C
|
192.168.1.x
|
G
|
10.2.x.x
|
10.2.0.1
|
|
default
|
A
|
|
D
|
10.3.150.x
|
E
|
10.3.100.x
|
E
|
|
10.3.x.x
|
10.3.0.1
|
|
default
|
A
|
|
E
|
192.168.1.x
*
|
H
|
10.3.150.x
|
H
|
|
10.3.100.x
|
10.3.100.1
|
|
default
|
D
|
|
F
|
10.1.20.x
|
10.1.20.1
|
default
|
B
|
|
G
|
10.3.100.x *
|
I
|
192.168.1.x
|
192.168.1.1
|
|
10.3.150.x *
|
I
|
|
default
|
C
|
|
H
|
192.168.1.x
|
I
|
10.3.100.x
|
10.3.100.2
|
10.3.150.x
|
10.3.150.1
|
|
default
|
E
|
|
I
|
10.3.100.x
|
H
|
192.168.1.x
|
192.168.1.2
|
|
10.3.150.x
|
10.3.150.2
|
|
default
|
G
|
The destinations with '*'
indicate destinations that shorten the normal route path through network
10.3.150.x.
Also
in this network since there are multiple possible paths, dynamic routing can be
used to provide alternate routing, if one router goes down.
