speed switched connections between endstations on Printer Switch 3rd Floor Printer Marketing Switch Engineering 2nd Floor Administration Router Switch Figure 6.10 An example of a VLAN Chapter 6 Local Area Networking different LAN segments. Users who want to belong to a particular broadcast domain do not have to be physically located on that LAN segment. VLANs provide a software-based, value-added function by enabling the cre- ation of a virtual broadcast domain, a shared LAN segment within a switched envi- ronment. Switching latencies on VLANs are typically one-tenth those of fast routers. However, routers are still required for inter-VLAN communications. Bridges Bridges, which entered the networking scene before routers, are used for connecting network segments (e.g., by creating the appearance of a single logical VLAN out of 5 to 10 individual clusters). A bridge can also be used to increase the number of com- puters on a network or to extend the distance of a segment beyond what the specifi- cations allow. Similarly, a bridge can be used for network segmentation in order to reduce traffic bottlenecks or to control the flow of network traffic. Bridges can con- nect similar as well as dissimilar networks, which is their main application. Bridges have several important functions: Learning-Wher a bridge is first connected to the network, it sends an announcement that says, "Hello. I'm your new bridge. What's your address?" All the other devices respond with, "Hello. Welcome to the neighborhood," along with their addresses. The bridge builds a table of local addresses, called the Media Access Control (MAC) sublayer addresses. The MAC sublayer (which is equivalent to OSI Layer 2) controls access to the shared transmission medium. It is responsible for making the data frames and putting bits in fields that make sense, and it works with the physical layer (Layer 1 of the OSI model). MAC standards, including IEEE 802.3, 802.4, and 802.5, define unique frame formats. Every NIC has a globally unique burned-in MAC address