Data Link Layer

Data Link Layer Overview

The data link layer provides the upper layers access to the network media. It is responsible for controlling access to the media, encapsulating packets into appropriately sized frames for the media used, physical addressing, the exchange of frames between nodes on the local network, and error detection.

Media Access Control

Layer 2, Local Area Network Technologies
and their Media Access Control Characteristics

Wireless Ethernet
Token Ring
Contention Based
(first come first serve)
Control Based
Collisions No Collisions
Send anytime
Wait for your turn
Physical Star Topology

Logical Multi-Access
or Bus Topology

Token Ring = Physical Star, Logical Ring Topology
FDDI = Physical Dual Ring, Logical Ring
efficient use of bandwidth
(send anytime)
Inefficient use of bandwidth
(you have to wait your turn)
CSMA/CD (ethernet)
CSMA/CA (wireless ethernet)
Token Passing

Control Based Access – Controlled access means that devices or nodes take turns in sequence. It is deterministic in that there is scheduled access of the medium. If one device is putting data on the network then no other device can. Well ordered and predictable throughput, can be an inefficient use of bandwidth, as a device has to wait it’s turn.

{loadposition adposition5}Contention Based Access – Contention based access is also called non-deterministic. This means that the devices on the network don’t need to take turns using shared media. However, to avoid total chaos, a Carrier Sense Multiple Access (CSMA) process is used to make sure the media is not in use before a device begins to transmit. Though devices attempt to make sure the media is not busy, data collisions still occur with contention based access. Also, as more nodes are added to the network, the probability of collisions increases.

CSMA-CD (Carrier Sense Multi-Access with Collision Detection) is a media access method in which an ethernet host detects if a signal is being transmitted. If no signal is detected on the wire, then the host will transmit. There does exist the possibility that two or more hosts may sense the absence of a signal and transmit at the same time. If this happens, there is a collision of signals.

CSMA-CA (Carrier Sense Multi-Access with Collision Avoidance) stands for Collision Sense Multiple Access with Collision Avoidance. This is used for wireless media access control. It uses a send and reply like the TCP three way hand shake, in this way it reserves the right to send before sending. After each message is sent the hosts associated to the wireless access point run a randomization algorithm which sets a random priority on who gets to send next. That along with many control fields help to mitigate some of the interferences and other radio related wireless problems.


Network Topologies

Physical Topology versus Logical Topology – The physical topology is the arrangement of devices (nodes) and how they are physically connected to the network. The logical topology is the way data is transferred from one device (node) to another regardless of how the devices are physically connected. It is also related to how each host sees other hosts on the network and how each host accesses the media. A network’s logical topology is not necessarily the same as its physical topology. For instance, in an Ethernet network, computers are often connected to a switch or hub forming a physical star topology, but logically the way the data travels is a bus or multi-access topology. In a Token Ring network, computers are connected to a MAU multistation access unit, forming a physical star, but logically information travels clockwise from host to host in a ring topology. In FDDI, the physical topology is a dual ring (expensive) and logically it is also a ring. For additional information see:

Bus or Multi-Access
Ring or Dual Ring
Full Mesh

Point to Point Topology – directly connects two nodes. All frames are placed on the media by one node and taken off by the other. It can be both a physical and logical topology. Physically it is two nodes directly connected. Logically it is two nodes virtually connected directly, but passing through a network. It does not include the other devices in separate locations, that the data travels through. In this way it forms a virtual circuit between the two nodes. A virtual Circuit is a logical connection between two nodes and end users do not notice the intermedate devices.

Multi Access Topology – means that the nodes are communicating on the same shared media. Only one node can use the media at a time, and every node sees every frame on the medium. Of course, only the node to which the frame is addressed actually processes the frame. When sharing media, CSMA/CD and token passing are used to reduce collisions.

Ring Topology – In a physical ring topology each device is connected to two neighboring devices creating a physical ring almost like a physical bus. In a logical ring topology each node receives a frame in turn, and if the frame is not addressed to that node, it passes it on. In a Token Ring network, a node cannot send data on the network unless it has the token, the token is then passed to the next node and so on in a logical ring. For more information see:

Data Link Layer Sub Layers

LLC – Logical Link Control sub layer – Helps interface with the upper layers meaning the Network layer. Logical Link Control (LLC) places information in the frame that identifies which Network layer protocol is being used. This information allows multiple Layer 3 protocols, such as IP, IPX, Apple Talk, and DECNet, to utilize the different types of local media and interfaces, like Ethernet, Token Ring, different WAN serial protocols and interfaces such as PPP, HDLC, etc. .

MAC – Media Access Control sub layer – Media Access Control provides data link layer addressing with source and destination MAC addresses. These addresses are 48 bit physical addresses, usually written in hexadecimal format and burned into the NIC. Media Access Control is also responsible for marking the beginning and the ending of a frame with a start-of-frame and an end-of-frame delimiter. For more information see: 

Layer 2 Frames

Layer 2 frame characteristics are similar to other layers. There is a header, the data payload, and the trailer. The specifics of the frame differ in regards to the type of frame in question. There are LAN layer 2 technology frames (Ethernet, Token Ring) and WAN layer 2 technology frames (PPP, HDLC). One of the main differences is that ethernet frames have source and destination MAC addresses in their frame headers and serial technologies like PPP and HDLC do not.

Communicating on a Network – Page 4

TCP/IP Overview

The TCP/IP Model is the implemented network protocol suite of the internet, the OSI Model is now considered a theoretical model because it was never caught on like TCP/IP but it has been a very influential model. Cisco uses both the OSI and the TCP/IP models to talk about how data is separated into pieces which are turned into smaller packages. This process is called encapsulation which happens from Layer 7 down to Layer 1. The encapsulated packets or packages travel across the network or the internet and are rebuilt – decapsulation) at the receiving host end. Decapsulation happens from the bottom up, Layer 1 to Layer 7

As data is built into packets or packages it is done in layers. By separating the networking process into layers developers and engineers are able to isolate the necessary functions for their products and not concern themselves with the entire networking architecture. In this way, a layered approach and the rules and protocols recommended by each layer, promote hardware compatibility, easier software development, and competition. By clearly separating the role of each layer networkers are able to easily troubleshoot network failures. It is common for network technicians to identify a layer 1 issue as an unplugged network cable. Similarly a computer which can ping an ip address but is unable to ping a domain name is operating fine at layer 3 but is having an issue at layer 7. See my video tutorials below for an explanation of network layer characteristics.

Video Tutorials

Communicating on a Network – Page 3

OSI Model Overview

As the individual packets are encapsulated from the segmented data each layer adds information to the packet in whats called a header. This header is called a PDU or Protocol Data Unit. The header or PDU has important information that is needed to get the packet from point A to point B. One important piece of information that is contained in the PDU headers is the source and destination addresses.


OSI Layer TCP/IP Layer PDU
7 Application Application Data
6. Presentation Data
5. Session Data
4. Transport Transport Segment
3. Network Internet Packet
2. Data Link Network Access Frame
1. Physical Bits

The addressing that is put into the header of the packets is very important because as the packets travel across the network and encounter networking devices, the devices will strip off the different header addresses which helps send the data to the proper destination.

Layer 7 – Application – Application Data
Layer 6 – Presentation – Formatting Data
Layer 5 – Session – Control Data
Layer 4 – Transport – Source and Destination Service – Port Numbers
Layer 3 – Network – Source and Destination Logical Addresses – IP addresses
Layer 2 – Data Link – Source and Destination Physical Addresses – MAC addresses
Layer 1 – Physical – Encoding, Timing and Bit Sequence

Source and destination MAC addresses handle the delivery of packets to hosts on a local area network. Every NIC or network interface card has a unique MAC address and using Ethernet, packets are delivered at the Network Access layer of the TCP/IP model. At this layer the PDU is called a frame and the source and destination addresses identify a single host. The Frame is stripped off and the packet is moves to the Network or Internet Layer. The MAC address is often called the physical address because it is burned into the NIC and not normally configured through software.

Source and destination IP addresses handle the delivery of packets to the correct network host. For TCP/IP networking every host must have an IP address which correctly identifies the network they are on and the host number they occupy in that network. Routers are able to read the source and destination addresses in the layer 3 packet header and forward the packet to the correct network. Later a switch will facilitate the delivery of the packet to the correct host NIC by means of the Layer 2 MAC address.

Source and destination ports identify the correct application or service that has made the request. For instance a port 80 request would mean that a web page is requested as opposed to an email which would be port 25.

The layered protocols, addresses, and source and destination addresses are very abstract because when we request a web page with a web browser we do not see all the protocols and network layers at work. We do not see individual packets just a finished web page in our browser. To help make these protocols and layers more concrete you can capture the packets as they arrive at your computer and look inside the different layer headers. To do this you need to use a program called Wireshark. Wireshark is very handy ‘packet sniffer,’ and is a free program to download. See my short video tutorial for a quick intro on how to use it.

Communicating on a Network – Page 2

Network communication does not happen without rules or protocols. In this class we will learn about the many protocols or rules that are necessary to send a message across the local area network as well as the internet. In order to have successful communication between people you have to follow social and cultural conventions, these are also called protocols. For instance, if I go into a restaurant and walk past the hostess and right up to the waitress, while she is helping another table and demand some food; all the while not wearing a shirt, will I be successful? Probably not, because I am not following the convention of waiting to be seated, greeting the hostess, following her to a seat, getting my menu and so on and so forth. The rules for proper behavior for the restaurant system. Computer communication follows similar rules called protocols.

Protocols that allow computers to send and receive messages over networks are called network protocols. The protocols necessary for network communication are grouped together in stacks called protocol suites. These groups of protocols work together hierarchically which is commonly referred to as working in a layered architecture. Protocol suites are responsible for the format of the message which is a specific syntax, the process by which network devices will send information, reporting errors, and the beginning and termination of communication. Although protocols can be proprietary to one or more products or vendors they are often times written to comply with industry standards maintained by international committees like the IEEE. In this way protocols can be interoperable with many other devices, protocols and standards. Network protocols give the rules that govern communication, “the what” of what needs to happen in order to communicate, not “the how” of how that communication will be carried out. In this way, many different makers of computer hardware and technology can create there own products in their own way, as long as they adhere to the standardized rules of communication. This is one of the examples of the benefits of a layered architecture, in that vendors do not have to write their own rules of communication, just adhere to the standards. Some of the benefits of using a layered architecture are: a common language to describe functions on specific layers, technology advancements on one layer does not effect the other layers (layer independence), specific layer requirements aid in the product design of how protocols interact with each other, interoperability allows for competition in the market.

The two most well know networking models are the TCP/IP and the OSI models. The OSI or open systems interconnect model is the most widely recognized reference model for developing network protocols and applications. The OSI model was created as an open international standard but it was not adopted at as fast as the TCP/IP internet model and as a result the OSI model is purely an influential reference model which helps in the creation of other protocols and services. The TCP/IP model is the model of the internet and is based on the TCP/IP protocol suite. The TCP/IP model was widely adopted. Both models ultimately reflect analogous network layers that follow the similar functions.

Layer OSI Model TCP/IP Model
7 Application Application
6 Presentation
5 Session
4 Transport Transport
3 Network Internet
2 Data link Network Access
1 Physical

The TCP/IP model has four layers but the Network Access Layer comprises the functions of both the Physical and Data Link layers in the OSI model. Similarly the Application layer of the TCP/IP model comprises the top three layers of the OSI model (Application, Presentation, and Session).

As data travels from a user’s computer (host) across the internet to another host the data is broken apart and built into “packets.” This process of building packets is called encapsulation. Encapsulation happens from the top down starting with a user’s network application like a web browser, the data is broken into segments and the packet is built in descending layers down to the Physical layer. The packets then travel across the internet and at the receiving end the data built back together, called decapsulation, starting at layer one and moving up the layers until the data is completely rebuilt at the application layer and presented to the user.

Network Communication – Page 1

Network Communication – Overview

One of the challenges in learning the Cisco CCNA is learning how to navigate the massive amounts of jargon or lingo. Like the word services which can also be worded as applications, or programs, and can also be called processes. Many processes and services run behind the scenes in an operating system, if it is a Linux OS we call them daemons. Now the test writers when they make up their multiple choice tests have to make it hard somehow, so you get the picture.

A great place to start learning networking is a discussion of network communication basics. Many texts compare computer communication to regular verbal communication where two people have a conversation and they take turns, one speaking and the other listening. This is analogous to computers sending and receiving information. When computers exchange information there is a sender (the source) and a receiver (the destination) over the medium or media. The media is name give to the copper wire, the fiber optic cable, or the radio waves if it is wireless. The sender and the receiver and the media make a channel for communication. The message or data is what travels over that channel. The message is divided into smaller pieces or segments. Commonly we refer to these as packets. Later in the curriculum the word ‘packet’ will receive a more specific meaning as a single part of the overall data segment. Phew!

Multiplexing is when different types of data can travel over the wire at the same time by interleaving the individual packets. This is multiple conversations going over the channel.

We can also distinguish between end devices on a network like a computer, an ip-phone, or a network printer and intermediary devices that connect the end devices. Like a switch, hub, router, firewall or wireless access point. On a network end devices are also called hosts or clients. Another type of host is a server. A server is a host that is running server software or server programs. This means that a server is listening for requests on specific ports and is able to respond or serve data when a request comes in. A computer can be a client a server or both at the same time.

Intermediary devices have a number of functions like regenerating and resending the data signals. For instance, data signals can only travel so far on a copper wire without having to be regenerated and resent. If the signals travel too far beyond specifications, without being regenerated, then the signal, in this case voltage will weaken and the end device will not be able to correctly decode the binary 1s and 0s. Intermediary devices also maintain information about paths through the network. For example routers know paths to different networks and switches know which end devices are connected to which ports on the switch. Intermediary devices can also report errors close or route data to other paths when there is failure on a link, prioritize messages according to QoS, and filter data according to access lists which can permit or deny the flow of data.

In class the question was asked, “What is the difference between a router and a switch?” A router interconnects and routes users to different networks and a switch connects users to a single network or LAN (unless it is configured with vlans). 

A LAN or local area network is a network that spans a specific area like a business, or a school. A lan is usually controlled and maintained by a single organization. The college where we have class is an example of a lan. At the college there are a lot of separate networks or subnets, many switches and routers but the entire college is in one location and under one administration so it is an example of a lan. Simply speaking if you have a bunch of computers and you network them together by connecting them to a switch and give them a common addressing or network protocol scheme then you have a lan. This could also be called an intranet or interior network in that it is interior to that organization only.

A WAN or wide are network is a network that connects lans across wide geographical distances. It is also the network that is formed between you and your lan and your ISP or internet service provider. If you have a Linksys wireless router at home or another brand you may notice that the physical ports on the back of the router are sometimes labelled LAN ports and WAN port. The lan ports connect to your home devices like computers and a network printer and the wan port connects to your modem or your ISP. In layman’s terms the wan port is what connects you to the internet.


Ethernet Overview

Ethernet is an important topic in the Cisco CCNA because network administrators typically oversee LANs (local area networks), and pretty much all LANs today use some form of Ethernet, whether it be copper Fast Ethernet, or fiber optic Gigabit Ethernet, or wireless Ethernet. Ethernet became what it is today, because it was cheap and easy to install. It continued to improve its standards and hardware (eg. hubs to switches), also it has remained backwards compatible with the ability to change physical implementations from wireless, to fiber, to copper, as well as change speeds and standards all within the same functional network.

Ethernet and Collision Domains

Early versions of Ethernet used coaxial cable (10Base5 Thicknet and 10Base2 Thinnet). The physical topology could be described as a single cable that all users connected to or tapped into, this was known as a physical bus or multi-access network. Logically Ethernet was also a bus, or multi-access network, all hosts on the network could see each other, and all packets as well. All users were essentially on the same cable or same collision domain. What characterizes an Ethernet collision domain is in a collision domain, when two users send packets at the same time, the result is a collision or spike of voltage on the wire and all sending of packets must cease for a short period of time.

If you have ten hosts connected to a hub using regular Ethernet cables (10BaseT, twisted pair) then all hosts comprise a single collision domain. If you connect to many hosts to a hub or extend the network by connecting hubs to more hubs and more hosts then network performance will decrease and collisions will increase. In this way, if you have ten hosts connected to a hub and that hub is connected to another hub with another 10 hosts, then that network also comprises just a single collision domain.

Collisions were exacerbated because of the fact that Ethernet was designed as a multi-access network, where all hosts see all other hosts and all packets as well. The number of hosts in the network, and the presence a broadcast packets coming from multiple hosts, would increase the chances for collisions to occur.

The advent of switches was a significant improvement for Ethernet and local area networks. Switches provide many important improvements to a network, including collision free networking and better bandwidth utilization. Whereas a hub receives a frame on one port and automatically forward it out of all other ports, in contrast a switch maintains a table or map of MAC addresses to switchports, and is able to switch a frame to the destination port where the destination MAC address resides. Only when a switch does not have the MAC address in its table, or if it is a layer 2 broadcast, will a switch forward a frame out of all ports except the one it came in on. Thus less frames are traveling on the network unnecessarily. Since traffic is sent to only one port, each port or link on a switch is considered its own collision domain. Thus, switches break apart or create collision domains as opposed to hubs which extend or grow collision domains. With the advent of full duplex communications, hosts connected to switches could both send and receive frames at the same time without collisions.

Ethernet and ARP

ARP stands for address resolution protocol and its function is to resolve IP addresses to MAC addresses at Layer 2. When a frame or “packet” needs to be delivered to a host on a local area network it needs to delivered to the host’s MAC address. If the sending host does not have the destination host’s MAC address in its ARP cache it will send an ARP broadcast packet requesting the MAC address from the destination host’s IP address. So a MAC address needs to be resolved from an IP address before a packet can be delivered on a local network. In this way, ARP is plays an important role in the functioning of local area networks. In the video below I demonstrate the ARP process using a command prompt and Wireshark.

For more information on ARP:
For more information on Multicast addresses:

Hexadecimal Notation, Counting and Conversion

The ability to convert binary to decimal and vice versa is important to the Cisco CCNA, but you must also know how to convert hexadecimal. Hexadecimal is a shorthand notation that is used in computers all the time. MAC addresses are written in hexadecimal notation like this: B3:A2:77:00:F1:C9. Their are hexadecimal color charts for HTML and the web like 0xFF0000 which equals the color red, and hexadecimal is used in programming as well.

In the Cisco CCNA, hexadecimal notation is introduced when learning about layer 2 physical addressing, or MAC addresses. MAC addresses are 48 bits long and are typically written in 6 character pairs separated by a colon or a dash (eg. B3:A2:77:00:F1:C9 or B3-A2-77-00-F1-C9), but they can also be written in pairs of six or groups of four (eg. B3A277:00F1C9 or B3A2:7700:F1C9). You will also find hexadecimal numbers with a “0x” prefix or a “h” suffix to indicate that the number is in hexadecimal notation.

Hexadecimal is a Base16 counting system because there are 16 characters or numbers (0,1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f) with “a” through “f”equaling the numbers 10 through 15. Since a single hexadecimal digit or character has 16 possible values we can equate one hexadecimal character with 4 bits (24 equals 16). This creates an easy conversion between a binary 8 bit number to a 2 digit hexadecimal number:

 10111000 in binary = 184 in decimal
1011 – 1000 (splits the 8 bits into two 4 bit nibbles)
1011 = 11 in decimal and B in hex
1000 = 8 in decimal and 8 in hex

0xB8 = 184 in decimal

Ethernet, Data Link, and Local Area Network Tips

  • Ethernet functions on both layer 2 the Data Link layer and layer 1 the Physical layer. In the TCP/IP model layers one and two from the OSI model are combined into the Network Access layer.
  • The Data Link layer has an upper and lower sublayer, the LLC and the MAC sub layers
  • 802.2 is the the LLC, logical link control sublayer. Its role is to function in software and identify the network layer protocol above it.
  • Ethernet at its core is CSMA/CD. Ethernet is collisions and collision detection.
  • Hubs cause collisions. Switches cause no collisions because each port is its own collision domain.
  • Source and destination MAC addresses change as a frame travels across networks. Source and destination IP addresses do not change.
  • You only need to send packets/frames to the gateway/router when you are trying to contact a different network.
  • Packets/frames destined for a host on the same network do not have to go through the router but are delivered directly to the destination host’s MAC address

Other Ethernet Topics

Ethernet as a WAN

MAC Address Structure

Ethernet Unicast, Multicast, and Broadcast

Ethernet Timing

10Mbps 100Mbps, 1000Mbps Ethernet

Physical Layer

Physical Layer Overview

The purpose of the Physical layer is to put digital bits on the media as encoded signals and to also receive encoded signals and turn them back into binary digits. Media at the Physical layer refers to either copper cables, fiber optic cables or wireless radio waves. Along with all the different types of cables the Physical layer also refers to the different connectors like RJ-45 connectors and ST/SC fiber optic connectors.

The Physical layer takes place in hardware as opposed to software, so instead of protocols and addressing the Physical layer is comprised of engineering standards defined by organizations like the IEEE, the ITU and the ISO.


Signaling is changing bits in to a form that can be transmitted over distances and read by connectors on each end. In general terms, 1’s and 0’s are represented on the medium as variations in voltage, the presence or absence of light and changes in radio waves. In this way, 1’s and 0’s are signaled by changes in amplitude, frequency, and phase.

Two early signaling standards were Manchester Encoding (Ethernet) and Non-Return Zero (NRZ). NRZ uses the voltage on the wire as a 1 or 0. Since this is a very simple method of signaling it can only be used in low speed links. Manchester Encoding uses segments register a change in signal that goes up or down. If the change is down then it will be a 0 if the change is up it will be a 1.


Encoding is used to improve efficiency and speed of data transmission. Code groups are used to encode bits into larger symbols prior to placing them on the media. For example, in the 4B/5B code group, four bit long codes are translated into five bit long symbols. One reason for this is that devices know that when they see a five byte symbol that doesn’t correspond to a four byte code or control code, the bits are an error or noise on the media. Another reason for this is that a long series of 1s could wear out or overheat media or network devices. Also, using code groups prevents data bits from accidentally matching a control signal, such as the bit pattern signaling the end of a frame.

Copper Media

The most commonly used network media uses copper wires to carry data between network devices. Copper media can refer to early ethernet implementations using coaxial cables like 10Base2 (Thinnet) and the predominant Fast Ethernet and Gigabit Ethernet using Cat5E UTP (unshielded twisted pair) cables. Unshielded twisted pair cables (UTP) use four twisted pairs of wires that are used for signaling and transmission, and coaxial cable uses a single copper conductor that is insulated by a shield. Cables used for networking all have requirements that are spelled out in Physical layer standards.

One problem with copper media is that it is susceptible to electromagnetic and radio interference from things such as motors, fluorescent lights, and radio transmitters. Interference problems can be solved by using different media, avoiding sources of interference when designing infrastructure, and properly handling and terminating cables. Unshielded twisted pair cables use the effect of “cancellation.” created by the twists in the cable pairs to resist electromagnetic interference.

Fiber Media

Fiber cabling uses glass or plastic fibers to let light signals travel from the source to the destination. Encoding schemes use light pulses for the signaling method. The speed with which light travels allows fiber optic cabling to deliver large data bandwidth rates and longer cabling runs. Downsides to fiber optic cabling is that it is more expensive than copper cabling and requires careful installation techniques to avoid sharp bends in the cable which will break the glass core. Because of its cost fiber cabling has been used mainly for backbones and vertical runs in networks. There are generally two types of fiber optic cabling, multimode cable and single mode cable. Single mode is more expensive, can be run farther distances, uses a laser as a light source, and has an 8 to 10 micron glass core. Multimode fiber uses a LED as its light signal, has a glass core of 50 to 60 microns, bounces the light inside of the cable, suffers from more light dispersion, and is cheaper than single-mode.


Wireless Media

Carries electromagnetic signals at radio and microwave frequencies and works well in open environments. Wireless media requires no physical access like copper cables and jacks, however, the easy open access that wireless provides also presents security risks.

  • IEEE 802.11 (WiFi) is considered a wireless LAN
  • IEEE 802.15 (WiPAN) is considered a wireless Personal Area Network, commonly known as “Bluetooth”
  • IEEE 802.16 (WiMAX) is considered a point-to-multipoint topology for wireless broadband access
  • 802.11a – 5 Ghz frequency, 54 Megabit per second,
  • 802.11b – 2.4 Ghz frequency, 11 Megabit per second,
  • 802.11g – 2.4 Ghz frequency, 54 Megabit per second,
  • 802.11n –  2.4 Ghz frequency, 100 Megabit per second

Media Connectors

EIA-TIA 568A and 568B are the unshielded twisted pair RJ-45 connector standards for wire colors used for pinouts for Ethernet straight-through and crossover cables. See the following diagrams:

A 568B “Straight Through” cable will have the following pin-outs on both ends of the cable

white/orange orange white/green blue white/blue green white/brown brown
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
white/orange orange white/green blue white/blue green white/brown brown


A 568A “Straight Through” cable will have the following pin-outs on both ends of the cable

white/green green white/orange blue white/blue orange white/brown brown
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
white/green green white/orange blue white/blue orange white/brown brown


A “Crossover” cable will have the 568B and the 568A standards on either ends of the cable
Notice: that pins 1&3 and 2&6 are crossed in a Fast Ethernet Crossover cable

white/orange orange white/green blue white/blue green white/brown brown
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
white/green green white/orange blue white/blue orange white/brown brown

Common Optical Fiber Connectors

Straight-Tip (ST) Trademarked by AT&T
-bayonet style connector used with multi-mode fiber
Subscriber Connector (SC)
– push-pull mechanism ensures positive insertion
– used with single-mode fiber
Lucent Connector (LC)
– small connector
– used with single-mode fiber
– supports multi-mode fiber

Common Fiber Termination and Splicing Errors
– media not precisely aligned together
End Gap
– Media does not completely touch
End Finish
– Dirt is present or the ends are not polished well enough

Optical Time Domain Reflectometer (OTDR)
– recommended test equipment
– injects a test pulse of light into the cable
– measures back scatter and reflection as a function of time
– calculates the approximate distance of detected faults

Field Test
– can be performed with a flashlight
– if light is visible at the other end cable is capable of passing light
– does not ensure performance
– used as a quick way to find broken fiber

IPv4 Addresses and Subnet Masks

The Format of an IPv4 Address

An IPv4 address can be written in two ways:

dotted decimal notation –
32-bit binary notation – 11000000.10101000.00000001.00000001

The address has 4 octets separated by periods and counted from let to right. There are three types of IPv4 addresses: a network address, a host address, and a broadcast address. In other words you could say a computer is on the /24 network (network address), and is using a host address of The address represents the ip address in dotted decimal notation. That same address in binary notation is 11000000.10101000.00001010.00000001. The 1(00000001) is in the 4th octet.

Converting Binary to Decimal and Vice Versa

The most popular, and (in my opinion) easiest way to convert a binary number to decimal is using a table like so:

128 64 32 16 8 4 2 1
0 0 1 1 1 0 0 1
0 + 0 + 32 + 16 + 8 + 0 + 0 + 1 = 57
Aside from knowing the table well enough to use it without writing it all out, there are various tricks for binary/decimal conversion. Personally, I’ve never found them very practical, but this doubling trick for converting binary to decimal, and this halving trick for converting decimal to binary are pretty cool.

Network Portion and the Host Portion of an IP Address and Subnet Mask

The network portion and the host portion of an ip address is defined its subnet mask. This process is easy if the subnet mask is classful meaning either:

Class C  –  or /24,
Class B  –  or /16,
Class A  –  or /8

So if the ip address is and the subnet mask is classful meaning then the 255s in the subnet mask tell you the network portion and the 0s tell you the host portion. For example, below the network portion is in red and the host portion is in black:
(So the network is, and the host is number 100)

Using the example above the first address in the network is the network address ( The last address in the network is the broadcast address (, and the host addresses in the network are the addresses between the network and the broadcast  ( –

The process is a little more difficult when a non-classful subnet mask is used. In this scenario binary conversion must be used to delineate the network and host portions of an address. Consider the following example: /27 or
Where are the network and host portions now?

To easily solve the question convert to binary:
11000000.10101000.00000001.01100100  =
11111111.11111111.11111111.11100000 =
(The 1s in the subnet mask identify the network portion, the 0s the host portion)

The network and host portions are still defined by the subnet mask, just more accurately by seeing the address and mask in binary and identifying the 1s and 0s. The question that you now have to ask yourself is, what is the networkaddress, broadcast address and host addresses if the subnet mask is To answer this question you need to, in binary, logically AND the ip address and subnet mask and you will get the network address. To understand this process and more see my video series on subnetting, ANDing and the Magic Number below.

Note: you have to have all subnet mask fields filled in.

3 Types of IP Addresses: Network Address, Host Address, and Broadcast Address

Network Address – The address by which we refer to the network
Uses the first address in the network,
The network address is reserved and is not usable by a host
All hosts in a network will have the same network address
All hosts in a network will have the same network bits or network portion

Broadcast Address – The address used to send data to all of the hosts on a network
Uses the highest (last) address in the network,
The broadcast address is reserved and is not usable by a host
The bits in host portion are all 1’s
Also called a directed broadcast

Host Address – The addresses assigned to the end devices in the network
Each and every device in the network needs a unique ip address,
The host addresses lie between the network and broadcast address

Public and Private Addressing

Private addresses are blocks of ip addresses that are not routable on the internet. The private address blocks are: to   ( /8) to   ( /12) to   ( /16)

Since private addresses are implemented on LANs behind a firewall different networks may use the same private address schemes. Private addressing requires Network Address Translation (NAT) in order to translate private addresses to public addresses for use on the internet. With this (NAT) technique, many hosts in a private network can channel all communications through a single public ip address allowing communicate over the internet.

Public Addresses are designed to be used by hosts that are publicly accessible from the internet. Public ip addresses are assigned by the InterNIC and consist of class-based network IDs called CIDR blocks.


Video Tutorial Series – IP Addresses, Binary Conversion, and Network Masks

In order to understanding of subnetting you need to be able to convert IP addresses from decimal to binary. Subnetting, subnetworks, and subnet masks only make sense from the perspective of binary. The reason you need to convert the addresses to binary is that it is the way routers find networks. Routers and computers find networks by ANDing IP addresses with the subnet masks. If you want to understand the logic behind the process you need to be able to see it from the perspective of the router. In the following video tutorials I lay out the simple process of converting IP addresses and subnet masks to binary. I also cover finding the network portion and host portion of a network or subnetwork mask. I recommend watching all of these videos as as my following series on the “Magic Number.”

Video Tutorial Series – Subnetting with the Magic Number – Parts 1 through 6

In this series of tutorials, I explain how you can easily find the network address, broadcast address, and first and last host addresses from any ip address and subnet mask combination. The ability to calculate subnets is the most important skill for success in the Cisco CCNA. The magic number trick will make that process a snap!

Unicast, Broadcast, and Multicast Messaging 

A message or packet sent to a unique ip host address is called a unicast message. A unicast message is a message addressed to a single unique host. By contrast a message or packet sent to a broadcast address is called a broadcast message. It is a message meant for all hosts on a network. A multicast message is a message sent to a multicast address, typically an address starting with 224 like An address that starts with 224 is a Class D address which is an address space reserved for multicasts. A multicast message is like a broadcast message in that most, or all, hosts on the network will open the packet and examine its contents before deciding whether or not to drop the message or send it up the layers for decapsulation.