Bridge different subnets

Hint: A bridge works on ISO layer 2 or the MAC address layer. It knows NOTHING about IP addresses, which are on layer 3.

You can create the bridge but nothing will pass. A network on: 192.168.1.0/24 will not pass traffic to a network on: 192.168.5.0/24 without an IP router (on layer 3). With just a bridge, the only packets that will pass are broadcasts.

Routers are used to glue together two different IP networks. In this case, it's done by setting up a static route through a gateway.

A really crude way to connect these two networks is to just enlarge the netmask to /16. Just change it from 255.255.255.0 to 255.255.0.0 and packets will pass in both directions. Traffic across the bridge will be limited to only those with destination MAC addresses that are across the bridge. Not the best of traffic control, but tolerable.

Someone suggested a transparent bridge. That will work with the /16 enlarged netmask. Note that you also have to increase the netmask on the client computahs to 255.255.0.0 or the clients will not "see" the machines on the other side of the network. Broadcast packets will go everywhere and across the bridge.

I can think of several ways to connect these networks. One is a router at the remote office with a static route to the remote office network IP block pointing to remote office router IP address (gateway). Another is to use a VPN which will assign additional IP addresses delivered from remote systems, to a local machine. Yech.

However, I don't see why you don't simply renumber the network so that all the machine fit in a single /24 IP block. If these networks are going to be connected permanently, you might was well make life easy and share the same /24 IP block. Now, if the remote office were connected via the internet instead of via a transparent bridge, then a router would certainly be necessary due to the limitations of a single IP addresss. However, with a transparent wireless bridge, that can pass multiple MAC addresses simultaneously, there's no benifit to using a router.

Incidentally, watch out for transparent bridges with a limited number of MAC addresses that may be bridged. The current products are 256 MAC addresses and up. However, some of the older devices will only do as few as 32. That's total MAC addresses for both ends including the wireless bridge MAC addresses.

Yes, read the following link for the info you seek.

Sorry, read down to transparent bridging for the info you want...

Robert....

Thanks for the detailed response. I appreciate that. I have a question about this though.

The learn table on both wireless bridges will contain the MAC addresses of all the nodes for there subnet and all the nodes that are in the other subnet by way of the bridge that it is connected to. Also, When a wireless WDS link (unlike a wired/Ethernet bridge) is set up between the two bridges all four address fields in the MAC header are used and transmitted. They are: The MAC address of the sender The MAC address of the final destination The MAC address of the sending Bridge The MAC address of the receiving Bridge

So, with this information won't the bridge in a fashion "route" a frame with a MAC address in the other subnet through the bridge because it knows by looking up in its learn table that the node at the destination MAC address is across the bridge?

Thanks for the help. I appreciate it. Todd

You need a router, or a PC with 2 NIC cards. We connected 50+ subnets in this way to segment the LAN.