Modeling the Network – Network Devices and their Interfaces

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When we try to identify ourselves within the Network, we usually run a commands like ipconfig /all (on ms windows platforms). Apart from information related to the computer we work on, the command displays a list of IP related devices – in our example – a wireless adapter, a network card and an ADSL modem, devices which allow us to communicate to a network. Each of these devices exhibit a physical address – six bytes in hexadecimal representation and, for two of them, for which the connection is active, an IP address. These devices, which are, to some extent, part of our workstation, serves as a network interface. This clarifies a first issue, IP addresses are not for computers, as the general belief goes, but for interfaces, like network cards,wireless adapters, firewire ports. Also, interfaces serve as end points for communication links, which may be UTP cables, coaxial cables or radio waves. To make a distinction between a workstation (or another physical
network element) and its interfaces, a physical device consisting of a processing unit with one or more interfaces will be called a network device.

The ipconfig /all command in Windows
The ipconfig /all command in Windows


A Gigabit network interface card
A Gigabit network interface card

A Network Interface Controller (NIC) or network card is a hardware device that handles an interface to a computer network and allows a network-capable device to access that network. The NIC exists on both the Physical and the Data Link layers of the OSI model. At layer 3 (Network) level NICs are identified by their IP (version 4 or 6) addresses.

Each NIC can be identified by a unique number, its MAC (Media Access Control) address (identifier) which is burned into its ROM (Read Only Memory). This address is globally unique, no two NICs may have the same address (identifier). The first 24 bits of the MAC address represent the so-called Organizationally Unique Identifier (OUI) and is manufacturer specific. Mac addresses are usually represented in hexadecimal format, as 6, 2-digit hexadecimal numbers, separated by “:” or “-”.

A couple of examples:
• 00-0D-60-F1-05-A7 – identifies an IBM (OUI = 00-0D-60) network card
• 00:30:6E:3B:ED:C3 – identifies an HP (OUI = 00:30:6E) network card

The current speed ratings for NICs range from 10Mbps (the original ethernet specification), 100Mbps for Fast ethernet and the new de facto standard of 1000Mbps or Gigabit ethernet. The 10Gbps NICs are very close to mass production, as well.

The actual connector to a NIC may vary from RJ45 (by far, the most widespread) to BNC (coaxial cable) or no physical connector at all, in the case of a wireless NIC.


A network hub or repeater hub is a device for connecting multiple twisted pair (TP) or fiber optic Ethernet devices and making them act as a single network segment. Hubs work at the Physical layer (layer 1) of the OSI model. This device is a form of multi-port repeater.

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The IBM 8242 Ethernet Hub

Hubs do not manage the traffic that comes through them; any packet entering any port is broadcast out on all other ports. Since every packet is being sent out through all other ports, packet collisions may result, affecting the efficiency of the data traffic. Hubs represent a cheap solution for interconnecting computers in small networks.


A repeater is an electronic device that receives a signal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances. Basicly,network repeaters regenerate incoming electrical, wireless or optical signals.

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Repeaters attempt to preserve signal integrity and extend the distance over which data can safely travel.

Actual network devices that serve as repeaters usually have some other name. Active hubs, for example,are repeaters. Active hubs are sometimes also called multiport repeaters, but more commonly they are just hubs. Other types of passive hubs are not repeaters.

Repeaters can be found both in wired and wireless networks.

Repeaters are used to lengthen individual network segments to form a larger extended network. That is, in the case of a wired network, repeaters allow a network to be constructed that exceeds the size limit of a single physical segment by allowing additional lengths of cable to be connected. There are some constraints,however. For a repeater to be used, both network segments must be identical-same network protocols for all layers, same media access control method, and the same physical transmission technique.

Repeaters do not have traffic management functions. They do not isolate collision domains or broadcast domains


Bridges are, in a sense, scaled down switches. Bridges make a simple do/don’t decision on which packets to send towards the segments they connect. Filtering is done based on the destination address of the packet.If packet’s destination is on the same segment where it originated, it is not forwarded. If it is destined for a station on another LAN, it is connected to a different bridge port and forwarded to that port.

A network bridge connects multiple network segments at the Data Link layer (layer 2) of the OSI model.

Using bridges over hubs or switches has several advantages:
• Isolate the collision domains
• Simple bridges are rather inexpensive
• Reduce the size of collision domain by microsegmentation in non-switched networks
• Transparent to protocols above the MAC layer
• Allow the introduction of management/performance information and access control
• LANs interconnected are separate, and physical constraints such as number of stations, repeaters and segment length don’t apply
• Help minimize bandwidth usage

On the other side, bridges have several disadvantages:
• Do not scale well to extremely large networks
• Do not limit (isolate) the scope of broadcasts (do not split the broadcast domain)
• Buffering and processing introduces delays
• Bridges are more expensive than repeaters or hubs


A network switch is a network device that connects different network segments. Switches operate mostly at the Data Link layer (layer 2) of the OSI model. Switches which operate at Network level or above are called multilayer switches.

There are three distinct functions of layer 2 switching:
• address learning
• forward/filter decisions
• loop avoidance

A rack mounted 24-port 3Com switch
A rack mounted 24-port 3Com switch

Although they do not isolate broadcast domains, switches isolate collision domain, significantly improving data traffic efficiency.

Switches are used to physically connect devices together. Multiple cables can be connected to a switch to enable networked devices to communicate with each other. Switches manage the flow of data across a network by only transmitting a received message to the device for which the message was intended. Each networked device connected to a switch can be identified using a MAC address, allowing the switch to
regulate the flow of traffic. This maximizes security and efficiency of the network.

Because of these features, a switch is often considered more “intelligent” than a network hub. Hubs neither provide security, or identification of connected devices. This means that messages have to be transmitted out of every port of the hub, greatly degrading the efficiency of the network.

Mid-to-large sized LANs contain a number of linked managed switches. Small office/home office configurations contain, in general, a single switch or an all-purpose convergence device such as a gateway,which provides access to small office/home broadband services such an ADSL router or a Wi-Fi router.

In switches intended for commercial use, built-in or modular interfaces make it possible to connect different types of networks, including Ethernet, Fibre Channel, RapidIO, ATM, ITU-T and 802.11.


A router is a network device used to forward data between computer networks. From a functional point of view, a router acts as a Network layer (layer 3) switch. In terms of network topology, routers isolate (split) both the collision domain and the broadcast domain. Routers operate in two different planes:
• the control plane – in which the router learns the outgoing interface that is most appropriate for forwarding specific packets to specific destinations
• the forwarding plane – responsible for the actual process of sending a packet received on a logical interface to an outbound logical interface.
Routers may provide connectivity within enterprises, between enterprises and the Internet, or between internet service providers’ (ISPs) networks. The largest routers (such as the Cisco CRS-1 or Juniper T1600) interconnect the various ISPs, or may be used in large enterprise networks. Smaller routers usually provide connectivity for typical home and office networks. Other networking solutions may be provided by a backbone
Wireless Distribution System (WDS), which avoids the costs of introducing networking cables into buildings.

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Wireless Access Points

A Wireless Access Point (WAP) is a device that allows wireless communication devices to connect to a wireless network using Wi-Fi, Bluetooth or related standards. The WAP usually connects to a wired network, and can relay data between the wireless devices (such as computers or printers) and wired devices on the network.

An AP is differentiated from a hotspot, which is the physical space where the wireless service is provided.

A small office network with one WAP
A small office network with one WAP

A typical example of a WAP is the wireless network interface controller (WNIC). It is a network card which connects to a radio-based computer network, unlike a regular network interface controller (NIC) which connects to a wire-based network such as token ring or ethernet. A WNIC, just like a NIC, works on the Layer 1 and Layer 2 of the OSI Model. A WNIC is an essential component for wireless desktop computer. This card uses an antenna to communicate through microwaves. A WNIC in a desktop computer usually is connected using the PCI bus. Other connectivity options are USB and PC card. Integrated WNICs are also available,(typically in Mini PCI/PCI Express Mini Card form).

A wireless NIC
A wireless NIC



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