Differences between Internet Connection Sharing (ICS) and bridging

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Network bridging is a very interesting feature on Windows systems. With bridging, we now have the ability to "connect" (or ""bridge") different networking technologies such as a wired Ethernet segment and a wireless 802.11a/b/g segment.

Consider the following example: You have two floors at your office, each one with a wired Ethernet network and you require to connect them together without drilling holes through the walls and adding wireless access points in order to keep the budget as low as possible.

You do however have one computer on each floor with one wireless network card along with an embedded network card on each computer's motherboard. In this case, you can bridge the two computers' wireless network cards between them automatically; the two networks are connected between each other. You should, however, keep in mind that bridging two networks together requires both networks to contain the same logical network addressing scheme.

In our example, if the first office had an IP addressing scheme of 192.168.10.0/24, then the second one will have to follow the same network addressing. A bridge does not route packets between logical networks, but it simply connects them together, making them one larger network, therefore the same rules that apply for a single network would have to apply in a bridged network.

Now here comes the tricky part: requirements and Internet Connection Sharing (ICS).

To create a bridge, you must select at least two network connections that are not being used by ICS. This effectively means that if you want to simply bridge two networks, you need two network connections (or interface cards) available on each network.

If you want to bridge two networks and also provide ICS services, then you'll need one additional network card on the network where the Internet service resides.

A common problem that users with wireless cards encounter when attempting to create a bridge is that the wireless card may seem to be working but the bridge is not created or the wireless network card seems to be unplugged.

In this case, your wireless network cards most probably does not support promiscuous mode. This is a very common problem amongst cheap wireless network cards. More information on this problem can be found at the following knowledge base article: Bridge may not work with a non-promiscuous mode network adapter.

Lastly, you can also visit my website, www.firewall.cx and SearchNetworking.com's Cisco Spotlight Series on bridging networks and applications where you'll find plenty of information on how to bridge your networks.