OK, more than you ever wanted to know...
The setup of the PCs is really no different than if they are on the same
hub, switch vlan, or directly connected via a twist cable. The configuration
of the PCs has to be done in one of two ways:
1. Set the PCs to the same ip subnet and mask (the default gateway doesn't
matter, it won't be used).
2. Set the PCs to entirely different ip subnets (or the same, as long as the
ip address is not identical or a broadcast address) and set the default
gateway the same as the ip address.
First Option:
-------------
Set the PCs to the same ip subnet and mask (the default gateway doesn't
matter, it won't be used).
For example:
PC#1:
IP: 10.100.1.20
MASK: 255.255.255.0
GW: 0.0.0.0 (or blank, depending on OS)
PC#2:
IP: 10.100.1.21
MASK: 255.255.255.0
GW: 0.0.0.0
The reason the default gateway doesn't matter is because both PC's are on
the same IP subnet. The default gateway is only used if you try to
communicate (ping) an ip subnet that is not local to the PC.
On PC#1, ping your loopback (ping localhost on Windows),
then ping your interface (ping 10.100.1.20) see above,
then ping the other PC (ping 10.100.1.21).
The reason for pinging your loopback and your own interface is to verify
that your IP stack is functioning and configured properly before you blame
the 'network'.
Since the two PCs don't actually use the IP addresses to communicate, you
can see what is actually going on by typing 'arp -a' in Windows to see the
local ARP cache. You should see the IP address of PC#2 (10.100.1.21) and
the MAC address. When you typed 'ping 10.100.1.21' on PC#1 (above), PC#1
actually:
a. Sent an ARP request on the wire (you will see this on your sniffer)
looking for the MAC address that answers to IP address 10.100.1.21.
b. Provided only one machine on this segment (hub, switch vlan, etc.) is
configured with this IP address, PC#2 will be the only PC to answer that ARP
request.
c. PC#1 will populate its local ARP cache with the IP address to MAC address
mapping
d. All communication with PC#2 will be with the MAC Address of PC#2 (not the
IP Address). The reason for the IP address is to make it easier for humans
to manage device addressing, the computers use only layer 1 (the cabling,
hub) and layer 2 (mac address, bridge) to send information to each other.
This is why you cannot ping an IP address on a different subnet
(192.168.255.1 for example). The PC will try to use a default gateway to
get there, which isn't configured and doesn't exist.
To illustrate this point a little better, let me explain the second option
for configuring the PCs:
Second Option:
--------------
Set the PCs to entirely different ip subnets (or the same, as long as the ip
address is not identical or a broadcast address) and set the default gateway
the same as the ip address.
For example:
PC#1:
IP: 192.168.255.26
MASK: 255.255.255.0
GW: 192.168.255.26
PC#2:
IP: 10.1.50.201
MASK: 255.0.0.0
GW: 10.1.50.201
Notice that the IP address and default gateway are identical on each
individual PC.
Now, why would you ever do this? To illustrate a point. If you would ping
PC#2 (10.1.50.201) from PC#1 (192.168.255.26), it will work!!! (Assuming you
started by verifying that you could ping localhost and your own interface).
Why does this work? Because, by setting the ip address and default gateway
the same, you tell the PC to ARP for everything. Even though the two PCs
are configured on different IP subnets, the PCs don't care because they
really use their MAC addresses to communicate. So, PC#1 sends and arp
request for 10.1.50.201 on the wire and PC#2 responds, PC#1 adds the MAC
address to its ARP cache and will send all further IP packets destined for
10.1.50.201 to the MAC address of PC#2.
Does this clear anything up? Here are sample configs for your two routers:
R1
--
! the following line is NOT necessary if you don't put ip addresses
! on the individual interfaces, like this config
!
no ip routing
!
interface ethernet0
no ip address
bridge-group 1
no shut
!
interface serial0
desc DCE
no ip address
clockrate 56000
bridge-group 1
no shut
!
bridge 1 protocol ieee
!
end
R2
--
! the following line is NOT necessary if you don't put ip addresses
! on the individual interfaces, like this config
!
no ip routing
!
interface ethernet0
no ip address
bridge-group 1
no shut
!
interface serial0
desc DTE
no ip address
bridge-group 1
no shut
!
bridge 1 protocol ieee
!
end
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=7181&t=7126
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