Multiple logical networks on same physical network ?

I would greatly appreciate if any one can help me the following situation

Environment: Windows 2003 servers with Gigabit NIC,GigaBit Switch


Situation is like this, we have 2 branches of office (same physical location) each office network is managed by different group, unfortunately both of them picked up same logical network (192.168.100.0 mask 255.255.255.0),now one of the machine in Branch-A needs to be accessed from Branch-B or vice versa,what are all the options without going for a router or re-routing ??Branch-A machine has dual NIC,one NIC is Free to use.If both logical networks are different (Branch-A & Branch- I would have connected Branch-A server to Branch-B network using Branch-A spare NIC,since they are same I couldn't do this,so decided to assign multiple IPs to Branch-B servers NIC and connect to same physical network.Secuirty is not an issue for us.


What is the downside of assigning 2 IP's to a single NIC on few of the servers(10) and using same switch for these multiple logical networks, in other words multiple logical networks on same physical network ?

Thanks in advance

Hello redg,

I'm sure you have thought of this, but just in case. Since the two networks use the same logical network, you could simply cascade the 2 networks switches via a crossover cable and resolve the IP conflicts that might arise using a DHCP server (if you were using static IPs). Now you apparently don't want to do this for physical or organizational reasons.

What is the downside of assigning 2 IP's to a single NIC on few of the servers(10) and using same switch for these multiple logical networks, in other words multiple logical networks on same physical network ?

As far as I know, Windows can handle multiple IPs on same NICs reasonably well. Say you setup a server with 192.168.100.20/24. and with another IP 10.1.1.20/24. When you ping some thing in the first subnet say 192.168.100.5, the source IP used for the outgoing packet will be 192.168.100.20, and when you ping the other subnet say 10.1.1.5, the source IP used is 10.1.1.20.

I have tried multiple logical networks on the same physical one before, but in a much simpler manner. The basic idea should work. However, (I guess) try to avoid setting any server on BranchB 192.168.100.0/24 subnet to the IP of the dual NIC branchA (first NIC) since this server could respond to ARP requests from BranchB causing frames to go to the wrong ports on the BranchB switch.

Another thing you might wana watch out for is if you are using dynamic updates to DNS or WINS server. Say you have a server named s1.abc.com. dynamic DNS updates could send the two IPs or the wrong IP to your DNS server, so s1.abc.com would map to the 2nd new IP rendering it unreachable by the routers.

Tell us what happens if you have the time.