A bridge is a device that separates two or more network segments within one logical network (e.g. a single IP subnet). The job of the bridge is to examine the destination of the data packets one at a time and to decide whether or not to pass the packets to the other side of the Ethernet segment. The result is a faster and quieter network with less collisions. In the OSI model, bridging acts in the first two layers, below the network layer.
The principle of ARPNAT is similar to NAT regarding IP networks, except that NAT works one layer deeper. Instead of translating the IP address, the router translates between the MAC hardware address on its side. If something on the wired side of the router makes an ARP request for the MAC address of an IP on the wireless side, then the router forwards the request as if it came from the router. When the response comes back, it mangles that too. Instead of passing back the real MAC (which lives on the wireless network), the router gives its own wired MAC address. Then, when it receives the frames for the IP addresses on the wireless network, it forwards them through. It does this to both sides of the bridge.
The following is an example of ARPNAT:
1. The Client PC asks what the MAC address of 192.168.1.67 is and asks to tell this to 192.168.1.5. This is a broadcast ARP request over wired Ethernet.
2. APC 5M (works as a STA) sees the ARP request and forwards the broadcast onto the wireless network as “What is the MAC address of 192.168.1.67. Tell it to 192.168.1.66” (this is APC 5M’s IP; remember that the Client was 192.168.1.5, so it is changed now).
3. APC 5M (works as an AP), which has the IP 192.168.1.67, responds with a result that is sent to 192.168.1.66. For example, the result is 00:19:23:66:12:56.
4. APC 5M (works as a STA) receives the response. It does some modifications and sends that request back to the Client PC as “192.168.1.67 is at 00:12:23:55:66:77“. This MAC is the MAC address of the APC 5M (STA) itself.
5. So, if the Client wants to talk to the AP, it will talk to APC 5M (STA), which will forward those frames to the AP.
When a bridge is installed between two networks, it gathers the packets from one network and repeats them to the other and vice-versa. This way, the nodes on one network can talk to the nodes on the other. Bridges also have features such as broadcast filtering, allowing for greater efficiency and reduced traffic. Therefore, in its most simple form, a bridge is a two-port network device that connects two network segments. Some advanced implementation of bridging can monitor traffic and determine which nodes are on which segment and later use this information while forwarding packets. The main factor to note is the fact that bridging takes place in the data-link layer of the OSI reference model.