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Networking Basics

Network devices

Let’s take a look at the network devices commonly found in today’s LANs..

Hubs

A hub serves as a central point to which all of the hosts in a network connect to. A Hub is an OSI Layer 1 device and has no concept of Ethernet frames or addressing. It simply receives a signal from one port and sends it out to all other ports. Here is an example 4-port Ethernet hub (source: Wikipedia):

hub

Today, hubs are considered obsolete and switches are commonly used instead.

Switches

Like hubs, a switch is used to connect multiple hosts together, but it has many advantages over a hub. Switch is an OSI Layer 2 device, which means that it can inspect received traffic and make forwarding decisions. Each port on a switch is a separate collision domain and can run in a full duplex mode (photo credit: Wikipedia).

Cisco switch

Routers

 A router is a device that routes packets from one network to another. A router is most commonly an OSI Layer 3 device. Routers divide broadcast domains and have traffic filtering capabilities.

The picture below shows a typical home router:

home router

In the next sections we will describe each of these devices in more detail.

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Unicast, multicast, and broadcast addresses

There are three types of Ethernet addresses:

1. unicast addresses

Unicast addresses represent a single LAN interface. A unicast frame will be sent to a specific device, not to a group of devices on the LAN:

Unicast Ethernet address

The unicast address will have the value of the MAC address of the destination device.

2. multicast addresses

Multicast addresses represent a group of devices in a LAN. A frame sent to a multicast address will be forwarded to a group of devices on the LAN:

Multicast Ethernet address

 

Multicast frames have a value of 1 in the least-significant bit of the first octet of the destination address. This helps a network switch to distinguish between unicast and multicast addresses. One example of an Ethernet multicast address would be 01:00:0C:CC:CC:CC, which is the address used by CDP (Cisco Discovery Protocol).

3. broadcast addresses

Broadcast addresses represent all device on the LAN. Frames sent to a broadcast address will be delivered to all devices on the LAN:

Broadcast Ethernet address

The broadcast address has the value of FFFF.FFFF.FFFF (all binary ones). The switch will flood broadcast frames out all ports except the port that it was received on.

 

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MAC & IP addresses

MAC address

A Media Access Control (MAC) address is a 48-bit (6 bytes) address that is used for communication between two hosts in an Ethernet environment. It is a hardware address, which means that it is stored in the firmware of the network card.

Every network card manufacturer gets a universally unique 3-byte code called the Organizationally Unique Identifier (OUI). Manufacturers agree to give all NICs a MAC address that begins with the assigned OUI. The manufacturer then assigns a unique value for the last 3 bytes, which ensures that every MAC address is globaly unique.

MAC addresses are usually written in the form of 12 hexadecimal digits. For example, consider the following MAC address:

D8-D3-85-EB-12-E3

Every hexadecimal character represents 4 bits, so the first six hexadecimal characters represent the vendor (Hewlett Packard in this case).

How to find out your own MAC address?

If you are using Windows, start the Command Prompt (Start – Programs – Accessories – Command Prompt). Type the ipconfig/all command and you should see a field called Physical Address under the Ethernet adapter settings:

ipconfig/all command

If you are using Linux, type the ifconfig command. You should see your MAC address referred to as HWaddress.

ifconfig mac address

IP address

An IP address is a 32-bit number that identifies a host on a network. Each device that wants to communicate with other devices on a TCP/IP network needs to have an IP address configured. For example, in order to access the Internet, your computer will need to have an IP address assigned (usually obtained by your router from the ISP).

An IP address is usually written in the form of four decimal numbers seperated by periods (e.g. 10.0.50.1). The first part of the address represents the network the device is on (e.g. 10.0.0.0), while the second part of the address identifies the host device (e.g. 10.0.50.1).

In contrast to MAC address, an IP address is a logical address. It can be configured manually or it can be obtained from a DHCP server.

NOTE
The term IP address is usually used for IPv4, which is the fourth version of the IP protocol. A newer version exists, IPv6, and uses 128-bit addressing.

 

Private IP addresses

There are three ranges of addresses that can be used in a private network (e.g. your home LAN). These addresses are not routable through the Internet.

Private addresses ranges are:

  • 10.0.0.0 – 10.255.255.255
  • 172.16.0.0 – 172.31.255.255
  • 192.168.0.0 – 192.168.255.255

How to find out your IP address

If you are using Windows, start the Command Prompt (Start – Programs – Accessories – Command Prompt). Enter the ipconfig command. You should see a field called IP Address:

ipconfig

Linux users:

Enter ifconfig. You should see a field called inet addr:

ifconfig ip address

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Ethernet frame

We have already learned that encapsulated data defined by the Network Access layer is called an Ethernet frame. An Ethernet frame starts with a header, which contains the source and destination MAC addresses, among other data. The middle part of the frame is the actual data. The frame ends with a field called Frame Check Sequence (FCS).

The Ethernet frame structure is defined in the IEEE 802.3 standard. Here is a graphical representation of an Ethernet frame and a description of each field in the frame:

ethernet frame

  • Preamble – informs the receiving system that a frame is starting and enables synchronisation.
  • SFD (Start Frame Delimiter) – signifies that the Destination MAC Address field begins with the next byte.
  • Destination MAC – identifies the receiving system.
  • Source MAC – identifies the sending system.
  • Type – defines the type of protocol inside the frame, for example IPv4 or IPv6.
  • Data and Pad – contains the payload data. Padding data is added to meet the minimum length requirement for this field (46 bytes).
  • FCS (Frame Check Sequence) – contains a 32-bit Cyclic Redundancy Check (CRC) which allows detection of corrupted data.

The FCS field is the only field present in the Ethernet trailer. It allows the receiver to discover whether errors occurred in the frame. Note that Ethernet only detects in-transit corruption of data – it does not attempt to recover a lost frame. Other higher level protocols (e.g. TCP) perform error recovery.

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Ethernet explained

Ethernet is the most used networking technology for LANs today. It defines wiring and signaling for the Physical layer of the OSI model. For the Data Link layer, it defines frame formats and protocols.

Ethernet is described as IEEE 802.3 standard. It uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access method and supports speeds up to 100 Gbps. It can use coaxial, twisted pair and fiber optic cables. Ethernet uses frames to with source and destination MAC addresses to deliver data.

NOTE
The term Ethernet LAN refers to a combination of computers, switches, and different kinds of cables that use the Ethernet standard to communicate over the network. It is by far the most popular LAN technology today.

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Cisco three-layer hierarchical model

Because networks can be extremely complicated, with multiple protocols and diverse technologies, Cisco has developed a layered hierarchical model for designing a reliable network infrastructure. This three-layer model helps you design, implement, and maintain a scalable, reliable, and cost-effective network. Each of layers has its own features and functionality, which reduces network complexity.

Here is an example of the Cisco hierarchical model:

cisco three layer hierarchical model

Here is a description of each layer:

  • Access – controls user and workgroup access to the resources on the network. This layer usually incorporates Layer 2 switches and access points that provide connectivity between workstations and servers. You can manage access control and policy, create separate collision domains, and implement port security at this layer.
  • Distribution – serves as the communication point between the access layer and the core. Its primary functions are to provide routing, filtering, and WAN access and to determine how packets can access the core. This layer determines the fastest way that network service requests are accessed – for example, how a file request is forwarded to a server – and, if necessary, forwards the request to the core layer. This layer usually consists of routers and multilayer switches.
  • Core – also referred to as the network backbone, this layer is responsible for transporting large amounts of traffic quickly. The core layer provides interconnectivity between distribution layer devices it usually consists of high speed devices, like high end routers and switches with redundant links.

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IEEE Ethernet standards

Ethernet is defined in a number of IEEE 802.3 standards. These standards define the physical and data-link layer specifications for Ethernet. The most important 802.3 standards are:

  • 10Base-T (IEEE 802.3) – 10 Mbps with category 3 unshielded twisted pair (UTP) wiring, up to 100 meters long.
  • 100Base-TX (IEEE 802.3u) – known as Fast Ethernet, uses category 5, 5E, or 6 UTP wiring, up to 100 meters long.
  • 100Base-FX (IEEE 802.3u) – a version of Fast Ethernet that uses multi-mode optical fiber. Up to 412 meters long.
  • 1000Base-CX (IEEE 802.3z) – uses copper twisted-pair cabling. Up to 25 meters long.
  • 1000Base-T (IEEE 802.3ab) – Gigabit Ethernet that uses Category 5 UTP wiring. Up to 100 meters long.
  • 1000Base-SX (IEEE 802.3z) – 1 Gigabit Ethernet running over multimode fiber-optic cable.
  • 1000Base-LX (IEEE 802.3z) – 1 Gigabit Ethernet running over single-mode fiber.
  • 10GBase-T (802.3.an) – 10 Gbps connections over category 5e, 6, and 7 UTP cables.

Notice how the first number in the name of the standard represents the speed of the network in megabits per second. The word base refers to baseband, meaning that the signals are transmitted without modulation. The last part of the standard name refers to the cabling used to carry signals. For example, 1000Base-T means that the speed of the network is up to 1000 Mbps, baseband signaling is used, and the twisted-pair cabling will be used (T stands for twisted-pair).

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Encapsulation

The term encapsulation is used to describe a process of adding headers and trailers around some data. This process can be explained with the four-layer TCP/IP model, with each step describing the role of the layer.  For example, here is what happens when you send an email using your favourite email program (such as Outlook or Thunderbird):

  1. the email is sent from the Application layer to the Transport layer.
  2. the Transport layer encapsulates the data and adds its own header with its own information, such as which port will be used and passes the data to the Internet layer
  3. the Internet layer encapsulates the received data and adds its own header, usually with information about the source and destination IP addresses. The Internet layer than passes the data to the Network Access layer
  4. the Network Access layer is the only layer that adds both a header and a trailer. The data is then sent through a physical network link.

Here is a graphical representation of how each layer add its own information:

encapsulation

Each packet (header + encapsulated data) defined by a particular layer has a specific name:

  • Frame – encapsulated data defined by the Network Access layer. A frame can have both a header and a trailer.
  • Packet – encapsulated data defined by the Network layer. A header contains the source and destination IP addresses.
  • Segment – encapsulated data as defined by the Transport layer. Information such as the source and destination ports or sequence and acknowledgment numbers are included in the header.

NOTE
The term decapsulation refers to the process of removing headers and trailers as data passes from lower to upper layers. This process happens on the computer that is receiving data.

 

Data encapsulation in the OSI model
Just like with the TCP/IP layers, each OSI layer asks for services from the next lower layer. The lower layer encapsulates the higher layer’s data between a header (Data Link protocols also add a trailer).

While the TCP/IP model uses terms like segment, packet and frame to refer to a data packet defined by a particular layer, the OSI model uses a different term: protocol data unit (PDU). A PDU represent a unit of data with headers and trailers for the particular layer, as well as the encapsulated data. Since the OSI model has 7 layers, PDUs are numbered from 1 to 7, with the Physical layer being the first one. For example, the term Layer 3 PDU refers to the data encapsulated at the Network layer of the OSI model.

Here is a graphical representation of all the PDUs in the OSI model:

Encapsulation PDUs

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Wide area network

The term wide area network is used to describe a network that spans multiple geographic locations. Consider an example. A company has two offices, one in London and one in Berlin. Both offices have a LAN. If the company connects these two LANs together using WAN technology, a WAN is created.

The key difference between LANs and WANs is that the company usually doesn’t own WAN infrastructure. A company usually leases WAN services from a service provider. A WAN spanning multiple cities could look something like this:

Wide Area Network example

Frame Relay, ATM and X.25 are different types of WAN technologies. The Internet can also be considered a WAN.

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Half duplex and full duplex

In telecommunication, a duplex communication system is a point-to-point system of two devices that can communicate with each other in both direction. These two types of duplex communication systems exist in Ethernet environments:

  • half-duplex – a port can send data only when it is not receiving data. In other words, it cannot send and receive data at the same time. Network hubs run in half-duplex mode in order to prevent collisions. Since hubs are rare in modern LANs, the half-duplex system is not widely used in Ethernet networks anymore.
  • full-duplex – all nodes can send and receive on their port at the same time. There are no collisions in full-duplex mode, but the host NIC and the switch port must support the full-duplex mode. Full-duplex Ethernet uses two pairs of wires at the same time instead of a single wire pair like half-duplex.

The following picture illustrates the concept:

Half full duplex example

Because hubs can only operate in half duplex, the switch and hub will negotiate to use half-duplex, which means that only one device can send data at the time. The workstation on the right supports full duplex, so the link between the switch and the workstation will use full duplex, with both devices sending data simultaneously.

Each NIC and switch port has a duplex setting. For all links between hosts and switches, or between switches, the full-duplex mode should be used. However, for all links connected to a LAN hub, the half-duplex mode should be used in order to prevent a duplex mismatch that could decrease network performance.

In Windows, you can set up duplex settings in the Properties window of your network adapter:

Windows duplex setting

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