RE: Subnet question [7:60711] posted 01/09/2003
Thank you very much for taking pains to right such a detailed explanation.
Thank you all for your answers they were very helpful.
From: Priscilla Oppenheimer [mailto:nobody@xxxxxxxxxxxxxx]
Sent: Thursday, January 09, 2003 12:36 PM
Subject: RE: Subnet question [7:60711]
You may not need virtual LANs. Real LANs solve the problem. :-) This is a
classic case of subnetting.
With DHCP, the client should get the right address when it broadcasts after
it moves, so there's no issue.
Leaving DHCP out of the picture, the need to ensure that a moved node can't
communicate is met simply by the way IP works.
Assume there's a client with this config:
address = 184.108.40.206
subnet mask = 255.255.255.0
default gateway = 220.127.116.11
Assume the client is physically sitting on the 18.104.22.168/24 network. When
it wants to send to nodes on the 22.214.171.124 network, it will compare its
address with the destination address, assume it's on the same subnet, and
send an ARP broadcast. The ARP broadcast won't reach the destination though,
which is on a different LAN, so it won't work.
(Make sure the router isn't configured for Proxy ARP. But even with Proxy
ARP, communication won't work. With Proxy ARP, the router could respond on
behalf of the destination on the 126.96.36.199 network. However that host
wouldn't be able to respond because it would assume that 188.8.131.52 is
Assume the client wishes to reach devices on the 184.108.40.206 or 220.127.116.11
network. It will compare its address with the destination address and decide
that it's not on the same subnet, so it needs to send to the default
gateway. It will send a broadcast for the default gateway, which won't work
because 10.10.1.1 is on a different LAN. Once again make sure Proxy ARP is
disabled. I'll leave it to the reader to figure out what would happen in
this case if Proxy ARP were enabled. :-)
The question of VLANs versus real LANs requires more info. How many router
ports to you have? Is each router port a subnet? Or do you plan to have
multiple subnets out one router port, in which case you need VLANs and
inter-VLAN routing on the router.
Nathan Nakao wrote:
> I'd probably use VLAN's.
> Conf t
> Int vlan 101
> Int vlan 102
> Int vlan 103
> Then setup the DHCP to assign IP addresses accordingly.
> Once that is done. Set the vlans to 101 for first floor, 102
> for second
> floor, and 103 for third floor.
> -----Original Message-----
> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On
> Behalf Of
> Tamhankar, Nitin
> Sent: Thursday, January 09, 2003 8:40 AM
> To: cisco@xxxxxxxxxxxxxx
> Subject: Subnet question [7:60711]
> This might be a very elementary question for some of you guys
> but I
> would appreciate the answer.
> If an office which has 3 different floors and has Cisco routers
> catalyst switches and windows environment. We need to configure
> it in
> such a way that each floor is on its own subnet for example
> floor1 18.104.22.168
> floor2 22.214.171.124
> floor3 126.96.36.199
> Also if a computer which has IP address in subnet 188.8.131.52 is
> from floor 1 to floor 2, it should not communicate with the
> unless its IP address is changed to one in 184.108.40.206 subnet.
> How it can be accomplished?
> Thank you
> [GroupStudy.com removed an attachment of type
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