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The 802.11 standard defines two modes of operation: Infrastructure mode, where all stations communicate through an access point, and ad-hoc mode, where stations communicate directly without the help of an intermediary. Ad-hoc mode can be useful for temporary peer-to-peer applications, such as when two laptop users want to exchange files over Wi-Fi.

Most businesses discourage use of ad-hoc mode because they prefer to enforce corporate security policy at the access point and gateway or switch connected to the access point. Users that communicate directly over ad-hoc mode essentially bypass those security measures. Ad-hoc mode can even be used as an attack method. For example, a Windows XP PC that previously associated to an access point with a given name (SSID) can be tricked into automatically re-associating in Ad-hoc mode to an attacker’s laptop that advertises that SSID. You avoid this attack by configuring XP (or any other wireless client software) to associate to preferred SSIDs in infrastructure mode only.

On the other hand, several vendors are now using ad-hoc mode as the foundation for building wireless mesh networks. Mesh networks have many applications, including outdoor metropolitan networks and mobile ad-hoc networks (MANETs). To learn more about Ad-hoc mode and its use in mesh networks, visit this NIST resource page. A standard for mesh networks is now under development, designated IEEE 802.11s.

In short, ad-hoc mode has many constructive uses, but unless you have a specific reason for enabling, your safest best today is to disable ad-hoc mode to prevent unwanted or risky associations.

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In general, a hub is the central part of a wheel where the spokes come together. The term is familiar to frequent fliers who travel through airport “hubs” to make connecting flights from one point to another. In data communications, a hub is a place of convergence where data arrives from one or more directions and is forwarded out in one or more other directions. A hub usually includes a switch of some kind. (And a product that is called a “switch” could usually be considered a hub as well.) The distinction seems to be that the hub is the place where data comes together and the switch is what determines how and where data is forwarded from the place where data comes together. Regarded in its switching aspects, a hub can also include a router.

1) In describing network topologies, a hub topology consists of a backbone (main circuit) to which a number of outgoing lines can be attached (”dropped”), each providing one or more connection port for device to attach to. For Internet users not connected to a local area network, this is the general topology used by your access provider. Other common network topologies are the bus network and the ring network. (Either of these could possibly feed into a hub network, using a bridge.)

2) As a network product, a hub may include a group of modem cards for dial-in users, a gateway card for connections to a local area network (for example, an Ethernet or a Token Ring), and a connection to a line (the main line in this example).

In telecommunication networks, a bridge is a product that connects a local area network (LAN) to another local area network that uses the same protocol (for example, Ethernet or Token Ring). You can envision a bridge as being a device that decides whether a message from you to someone else is going to the local area network in your building or to someone on the local area network in the building across the street. A bridge examines each message on a LAN, “passing” those known to be within the same LAN, and forwarding those known to be on the other interconnected LAN (or LANs).

In bridging networks, computer or node addresses have no specific relationship to location. For this reason, messages are sent out to every address on the network and accepted only by the intended destination node. Bridges learn which addresses are on which network and develop a learning table so that subsequent messages can be forwarded to the right network.

Bridging networks are generally always interconnected local area networks since broadcasting every message to all possible destinations would flood a larger network with unnecessary traffic. For this reason, router networks such as the Internet use a scheme that assigns addresses to nodes so that a message or packet can be forwarded only in one general direction rather than forwarded in all directions.

A bridge works at the data-link (physical network) level of a network, copying a data frame from one network to the next network along the communications path.

A bridge is sometimes combined with a router in a product called a brouter.

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