Physical Components

An important aspect of understanding the network is knowing the hardware components. We are going to have a look at the major components of a LAN:

• Network Interface Card (NIC)
• Hub
• Switch
• Router

Network Interface Card

The NIC is the adapter card used to connect a device to the LAN. It allows us to send and receive messages to and from the network. A NIC has a unique MAC (media access control) address that provides a unique identification of each device.

The MAC address is a 12-byte hexadecimal number uniquely assigned to an Ethernet network card. This address can be changed by a network driver dynamically (as is the case with DECnet systems, a network developed by Digital Equipment), but usually the MAC address is not changed.

Hub

Multiple devices can easily be connected with the help of a hub. A hub is a connectivity device that attaches multiple devices to a LAN. Each device typically connects via a UTP (Unshielded Twisted Pair) cable to a port on the hub. You may have already heard about the RJ-45 (Registered Jack-45) connector. This is one of the possible port types on a hub, but a hub can also support other cable types. A hub can have anything from four ports to 24. In a large network, multiple hubs are mounted in a cabinet and support hundreds of connections.

Switch

Switches separate networks into segments. Compared to a hub, a switch is a more intelligent device. The switch stores the MAC addresses of devices that are connected to its ports in lookup tables. These lookup tables allow the switch to filter network messages, and, unlike the hub, avoid forwarding messages to every port. This eliminates possible collisions, and a better performing network can be achieved.

Router

A router is an intermediary network device that connects multiple physical networks. With many hosts it can be useful to split a LAN into separate portions, or subnets. The advantages of subnets are:

• Performance is improved by reducing broadcasts, which is when a message is sent to all nodes in a network. With subnets, a message is only sent to the nodes in the appropriate subnet.
• The capability of restricting users to particular subnets offers security benefits.
• Smaller subnets are easier to manage than one large network.
• Subnets allow a single network to span several locations.
  
  
  A router holds a routing table that lists the ways that particular networks can be reached. There will often be several different routes from one network to another, but one of these will be the best, and it is that one that is described in the routing table. Routers communicate using routing protocols that discover other routers on the network, and support the exchange of information about networks attached to each router.