My very own N+ Blog!!

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..If no-one else minds I'm going to use this thread to convey what I think I've learnt and to verify whether my knowledge is factual,non-factual,completely misunderstood or totally and completely above my station..starting with......

Mals OSI interpretation


OSI=open systems interconnect

A conceptual model consisting of SEVEN layers,numbered from top to bottom 7-1.This is how Mal see's it..

Layer 7 Application-the layer closest to the end user,not associated with applications we use like 'Word' or 'Internet Explorer' but referring to the actual application across a network by means of certain protocols that reside at this level.These protocols include (and I'm reciting this from memory!)....FTP,TFTP,DNS,POP3,SMTP,HTTP and TELNET.

Layer 6 Presentation-exactly as it says on the tin!!Data is manipulated in this layer,compression,encoding and translation of character sets happen in this layer and of course on the recieving end decompression,re-encoding and translation.

Layer 5 Session-The key phrase here is 'END TO END COMMUNICATION',this is where the communication between two or more hosts is maintained and where the communication (session) can be 'torn down',it is arguably the singlemost important layer of all,in my opinion anyway.NN SAP resides here.

Now for the lower layers (yes,the top 3 are upper layers)

Layer 4 Transport-Now things really start to happen with our data.This layer manipulates our data into 'SEGMENTS' and here is where we find the TCP/UDP protocols residing and I'm open to correction here but I believe IPX/SPX might be round here somewhere..!

Layer 3 Network-All of our ROUTING is handled here,with routers and layer 3 switches being part of the network layer.Here we see the SEGMENTS passed down from the transport layer turned into PACKETS.Protocols here include IP,ICMP,IGMP,ARP AND RARP (IP to MAC and MAC to IP resolution)

Layer 2 Data Link-an oddball layer really,since it is actually two sub layers rolled into one.The LLC (logical link control)layer talks with our newtork layer and deals with the IP address and flow control while our second sub layer-MAC-deals with physical addressing (the MAC address no less!) and talks to the lower 'PHYSICAL' layer.Error checking (NOT ERROR CORRECTION) takes place at this layer using CRC (cyclic redundancy check).We see Bridges,switches and WAP's here.Our packets passed down from the network layer are now turned into FRAMES and made ready for the last layer...

Layer 1 Physical-exactly that,all the physical aspects of the communication are residing here including hubs,MsAU's,transexuals-no makes that transcievers,NIC's and repeaters,cables and here we have our frames broken down into the now familiar 1's & 0's,or BITS(signal encoding).The data is transmitted across our physical link and ends up at layer 1 on the recieving end,where this whole mullarkey repeats itself in the exact reversre order..

not bad for a welshy huh?

OK,lets break things down and makes this lot a little easier to digest..

Use some sort of nmemonic All Poeple Seem To Need Data Processing,that floats my boat.
Remember S P F B,segments into packets into frames into bits.
Look at the application layer protocols,all directly linked to applications we can use,web browsers,email,remote services,file transfers..kapiche?
Presentation has no real protocols that we are concerned with,but presenting a decent picture might include .mpeg .jpeg .gif....all found here...kapiche?
Session...we all like a good session down the pub don't we?well to keep the session going the pub needs to remain open!...kapiche?
Transport...lets box this stuff up ready for delivery and we can either phone the recipient to say it's on its way (TCP) or just send the bugger regardless (UDP-connectionless)
Network..choose the quickest route and lets make sure we have the right address....kapiche?
Data Link..ok,well lets check for damage and make sure we use a postcode...kapiche?
Physical...posties on the case...package delivered...kapiche?


By the way,feel free to add your own content and feel even more free to correct me on mine..

 

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Why not - great idea !! Pinned

 

Looks good from here Mal...

 

Topologies

In my world topologies are either visible,or invisible!And you ask?"Shut up you great Welsh nincompoop,what the jeebers are you talking about?"

Good question.Topologies are presented to simpletons like myself in two formats and those formats are 1)Physical and 2)Logical.The physical topology is one that you actually see (in most cases abyways but i'll explain that a bit later),it consists of the things you actually see,the hardware,the cables,the hubs,switches and bosses daughter (ok,maybe not the bosses daughter but a payrise is a payrise right?).The logical topology is invisible,it is the way in which data is manipulated across the physical medium,it is the data flow and the way in which it flows determines the logical layout.

Physical topolgies..

Bus
Star
Ring
Mesh
Hybrid

The bus has a single backbone,is inexpensive (by comparison) as it uses less cabling.The physical bus is a bummer for us budding techies since it has a fault tolerance rating of zilch,if the network develops a fault the whole kit and kaboodle will implode on itself in less than 20 parsecs.Scalability (the ability to add new hosts with minimal disruption) is somewhere over the rainbow,you are going to have to take much verbal and possibly physical abuse when you add a new host since you may bring the entire network down in order to do so.

The star topology on the other hand has a comfortably high fault tolerance as only the host attached to the faulty cable will lose connectivity,unless of course you are unlucky enough to have a hub break down,in which case the star topology reveals it's weakness-the central point of failure (the hub dummy!).Think of the star topology as a wagon wheel (not the biscuit fools!),with the spokes protruding out of the central hub (on the end of which you can add a host),if one spoke breaks you have a minor problem,if the central hub breaks however you have a major connectivity meltdown.Star topolgies are relatively cheap to install and maintain and are highly scalable.

Ring topologies are layed out as a physical star (usually) and use an MAU or MsAU (same thing) or Multi Station Access Unit.Though the physical layout is that of a star topogy,the way the data is manipulated across the medium is by a method called token passing,hence a token ring network.The 'ring' is actually created physically within the MAU (MsAU) itself.FDDI (fiddy) is a dual ring technology and makes up for the single rings lack of fault tolerance,effectively a single ring network will fail entirly if a cable fails,whilst FDDI uses two counter-rotating rings that provide a 'backup' to one another,sort of!.Scalability-low.

Mesh topolgies are the mutts nuts in terms of reliability,since each node has at least 2 connections to every other node on the network,so if one connection path fails the mesh will allow the use of a second path to continue the flow of data.Draw three circles on a peice of paper,now connect all the circles together with straight lines so that every circle connects with a line to every other circle and bingo,you have a mesh network.Now try 10 circles and do the same thing!messy huh?The drawback of a full mesh network is the amount of connections you would need to accomplish this physical topology.To calculate how many connections you would need simply use the algebriac formula n(n-1)/2,feel free to slap me at any time.n is the number of hosts,for an example i'll assume we have 10 hosts,so n=10.Multiply 10 by n-1 (10-1=9) which gives us 90 and now divide by 2,thats 45 in case you're wondering,45 lovely connections and a carpet of cat5.Mesh is COMPLICATED if it is not administered sensibly,thus partial mesh and hybrid mesh topologies are a suitable alternative.It is expensive by comparison and is moderate in scalability (full mesh is anyways).

Hybrids?a mish mash of the other topolgies to create the perfect being,doctor.

There's a massive amount I could type about the logical topology,but thats not for now,thats something you will need to learn alongside the physical aspects of your network.What appears to be a star is not a star,it's a ring......!!

 

One thing that is most definatley being drummed into my empty head is the importance of the OSI model and it’s implementation,both physically and logically into the networking environments I come across and the theory that makes up the CompTIA N+ syllabus.Osi has many facets,many aspects that we have to comsider,ranging from the transparent processes that take place at each layer to the visible aspects of the process,which is what this little nugget of knowledge is going to convey…


Devices on the OSI Model.


Layer 3-Network.

I’m happy to say that only two devices that we need to know about are resident on the Network layer of the OSI model and they are 1)Routers and 2)Layer 3 switches..

Routers

Since the network layer is responsible for routing it makes sense that we find the router here,duh!Routers connect different networks together using addresses of the same protocol,IP for example-IP addreses are used to ‘route’ the packets from one destination to another.A router must have at least TWO interfaces,think about it!Incoming packets and outbound packets,commonly known as interfaces E0 and E1,although a router may have more than two interfaces,be aware of that one.Routers use a routing table which can be DYNAMIC or STATIC.Todays technology predominantly uses the DYNAMIC table but STATIC tables which are manually configured are still out there,remember this.The ROUTING TABLE,put simply is the routers A to Z map of where the packets can be sent to reach the destination host IP,yup!Its a map,of sorts.Don’t get Routers too confused with GATEWAYS though,where they can be integrated together in terminology the GATEWAY is capable of connecting networks that use different hardware and translates different protocols in the process.More about Gateways in another post.Routers WILL NOT forward broadcasts.

Layer 3 switch

The layer 3 switch is a switch that has routing capabilities and is becoming more common in todays networks.Since it has routing capabilities it is found on the Network layer.It is NOT to be confused with a Leyer 2 switch people!!!It is NOT the same thing,however should you hear the term ‘switch’ in your studies and more importantly in your exam,it will be a LAYER 2 switch.Reference to a layer 3 switch will be referred to as exactly that,Layer 3 switch.Thats all I know and about all I need to know about Layer 3 switches,unless I’m wrong in which case feel free to slap me and correct me.

Layer 2-Data Link

The data link layer,the oddball,the layer of two layers,the meatfeast salad bonanza club sandwich layer-split into two sub layers,but that’s another subject.Here we find the Layer 2 switch and Bridges.Let’s just spend a moment looking at this sensibly,just for a change,the data link layer splits into the LLC and MAC sub layers,so it makes sense that the devices that work on this layer are dependant on MAC addressing,this is how they ‘post’ their respective messages,by MAC.


Switches

Switches are clever little things,since they have the intelligence to NOT knock every door on the street to find the right address for delivery.They do this by compiling a MAC Address Table that remembers which port referes to which MAC address on the network.For example port 1 might be linked to a NIC with the MAC address a1-a1-a1-a1-a1-a1 so our switch only sends packets addresses to that MAC over port1,get it?This makes for a highly efficient delivery system and one that has another benefit-each port (or at least each node wired to a port) effectively has it’s very own collision domain.Switches send broadcasts.They will use one of three methods of handling data and they are FLOOD,FILTER and FORWARD,remember this.FILTER will drop a packet that is sent from the same segment,FLOOD will forward the packet to all destinations in an attempt to find the receiving host and FORWARD will simply forward the packet to it’s destination,I think!


Bridges

A bridge is used to split up a network into two separate networks,in a scenario where network traffic is becoming a problem and slowing the whole network down.The bridge has a MAC address table for each side of the bridge and will only forward packets across the bridge if the MAC address is on the opposite side,put very simply of course!It can connect networks that use different media too.It will forward broadcasts.



Layer 1-Physical

NIC-the NETWORK INTERFACE CARD,the device to which your network media will connect.This is where you would plug the RJ45 or LC or whatever connector into your PC,SERVER or workstation.It encodes the data into the raw format of 1’s and 0’s and transmits the data across the network media to the receiving port.

HUB-a dumb switch in effect since it will forward incoming packets to ALL ports in order to find the receiving host.This is ok for a small network,but larger networks would grind to a halt with just HUBS in place,since these broadcasts would cause considerable bottlenecks in the network.There are however ACTIVE and PASSIVE hubs,with active hubs performing the job of amplifying the signal of incoming packets and passive hubs simply broadcasting without amplification.Active hubs are near identical to REPEATERS in this respect and are commonly called MULTIPORT REPEATERS.

TRANSCIEVERS- a device that transmits and receives data,that’s it!

MAU or MsAU-the token rings equivalent of a hub,put briefly.This unit,though near-identical to the appearance of a hub,holds the secret to the token ring network,it is where the ring is actually,physically,implemented.

Apologies in advance for my mistakes,and I know there will be a few,dammit!But all this stuff is 95% written from memory,with the occasional reference to my All IN ONE book in those far off moments that require inspiration.

 

As much as we loathe to do it,we sadists just HAVE to learn those tables of statistics that look like a bowl of badly cooked spaghetti at first glance,but we must learn them.This little number is no exception and one that took a few days to master ( and about 3 seconds to get wrong I’ll add!).So without further ado I present…


Common Ports

Port 20------FTP data port
Port 21------FTP listening port
Port 22------SSH secure shell service
Port 23------TELNET remote access and admin service
Port 25------SMTP simple mail transfer protocol (sending email)
Port 53------DNS domain name resolution protocol
Port 69------TFTP ensecure FTP service-no authorization required
Port 80------HTTP web services
Port 110-----POP3 post office protocol for receiving email
Port 119-----NNTP net news transfer protocol,self explanatory
Port 123-----NTP network TIME protocol
Port 143-----IMAP typically IMAP4,alternative to POP3
Port 443-----HTTPS secure web service implementing SSH

Now that’s not too shabby,if a little brief,but this is what you are expected to know and you can bet your arse you WILL be tested on this stuff in your exam,I’m preparing for it and so should you!The way to learn this table is the write it down and KEEP writing it down until it sticks.Once it’s in there,it prettymuch stays there since us poor N+ wannabes have little else to think about right?

 

You think this stuff is easy?Shoot me if disagree but I'm going for a pint tonight with some friends,whom after listening to me recite all these tables and statistics will set fire to my man-bits and book me a table for one in the India Gate (ask Boyce about the India Gate).Here we go,this time it's IEEE,the Institute of Electrical and Electronics Engineers,you see a pattern forming yet?


IEEE standards (the ones we should know anyways!)



IEEE 802.1 Internetworking
IEEE 802.2 LLC (data link layer)
IEEE 802.3 Ethernet
IEEE 802.3i 10baseT
IEEE 802.3u 100baseT
IEEE 802.3z 1000baseT (gigabit ehternet)
IEEE 802.3ae 10000baseT(10 gigabit ethernet)
IEEE 802.5 Token Ring
IEEE 802.11 Wireless (a,b&g standards)

Thats not too much to learn really,I don't think so anyways,but to make sure I cram it in my head,I made some flashcards,gave them to the kids and did a deal whereby they can ask me anytime,anywhere what these standards are and if I get it wrong I pay them 50p every time.So far I've cost myself....£2.50,gutted,but learning fast.

 

It's awesome!

 

Er - from memory - you need to remember that this was a snapshot from one point in time.

IIRC many of the 802.3 standards have now been rolled into the main standard.

Harry.

 

Good point Harry.We should know that the IEEE standards can and will be modified,canned,added to and deleted from time to time.For the purpose of N+ I have tried to memorise those that seem most relevant to the objectives but there are many more,MANY MORE IEEE standards,we just wouldn't come across them at this point.

 

Wow, I can teach myself N+ with this. You better pass! :P

 

...hopefully I can teach myself to pass it too!!!

Anyways,onwards and on to the next little minefield of stats,the newest addition to the objectives...

Wireless...

Lets just point something out for a moment and I don't mean my John Thomas,I'm going to regurgitate what I know SO FAR about wireless,albeit a little limited but there will be scope for more on later posts,when it really has all sunk in.So,don't fill in the gaps,rather repair the cracks and I'll be a little wiser for it,I think.

IEEE have allocated wireless its own designated standard-IEEE 802.11 (IEEE 802.11x is a very common way of seeing it,where the 'x' can present certain sub categories such as 802.11a and 802.11b etc etc).It is a widely implemented and increasingly popular method of data transfer and comes in many flavours,however for the purpose of the N+ and for the purpose of my sanity I am going to try and cover the ones we need to know about,namely 80211.a,b,g and n,bluetooth and Irda.



802.11a

This standard runs at a frequency of 5Ghz (gigahertz)has a maximum throughput of 54Mbps (it is ACTUALLY rated at 56Mbps but is accepted at 54Mbps,din't know why,but it is so...it is).It has an indoor range of about 100 feet or 30 metres and is less prone to interference from everday devices like cell phones and microwave,I'll explain that bit later.It uses the OFDM method of transmission.


802.11b


This standard runs at the 2.4Ghz frequency,is widely distributed but fast becoming less popular than its succesor,the 802.11g standard.It has a relativley slow transmission speed of only 11Mbps and an indoor range of 100 feet or 30 metres.It uses the DSSS method of transmission.


802.11g


This standard uses the 2.4Ghz frequency,has a maximum throughput of 54 Mbps and an indoor range of 100 feet or 30 metres.It uses the DSSS method of transmission.

802.11n


This standard can use BOTH 2.4Ghz and 5Ghz frequencies,has a maximum speed of 540 Mbps (WOW!) and 100 feet indoor range (30 metres but we know that by now don't we?).

As far as my memory allows I recall that the b nd g standards are compatible with each other,but the a is only compatible with b using specific hardware,feel free to enlighten me on this one.


Bluetooth


A very popular standard with mobile phone addicts,but is very limited outside of it's intended use in a PAN or Personal Area Network.Again it uses the 2.4 Ghz frequency,has a maximum throughput of 720Kbps (Mbps would nice though) and a range of 1m.It uses the FHSS method of transmission.


IrDA


A less popular transmission method but useful nonetheless,this is the infra red standard having two flavours-v1.0 with its throughput of a meesly 115Kbps and its bigger brother the v1.1 with a whopping 4Mbps rate,oooooooo.It is line of sight.


Now,get that lot in your head and remember the other little snippets that I think are going to appear on the exam...

Wireless security is either WEP or WPA.WEP (wired equivalent privacy) uses up to a 128 bit security key for authentication but is considered less secure compared to WPA,which uses advanced encryption and closes many of the holes in WEP.

Transmitting radio waves requires an antenna and we have two types that we should discuss,albeit very briefly,they are YAGI and OMNI.Yagi antennas are 'point to point' in as much as they must be perfectly aligned on the reciever to be of maximum efficiency,if to work at all in fact.They are used to point to WAPs,or Wireless Acces Points.The signal strength is strongest in a straight line.The second type is the OMNI antenna and is one that transmits signals in ALL directions,like a radio station transmitter does for example.OMNI antennas are used for WAPs.

The 2.4Ghz frequency is widely implemented across several wireless standard,but you should be aware that there are devices that may well cause havoc with your wireless network-microwave ovens and cordless phones!Both of these devices share the same 2.4Ghz frequency and can severley impede the performance of a wirelss network that uses the same 2.4Ghz frequency,be wise to that one.

Radio waves do not like solid objects so be wary of the environment in which you set up a wireless network and use your common sense to answer any question that relates to environment,I did get one on my practice exam software but I'll be jiggered if I can remember the question right now.

Ad-HOC and INFRASTRUCTURE made reaaly,really easy....Ad HOC uses no WAP,INFRASTRUCTURE uses WAPs.

 

Ok,I sat a practice exam last night that presented the question of which modulation method 802.11g uses.According to my study material and certainly from memory I understand (at this point in time) that 802.11g uses DSSS as a transmission method,however my exam result marked this as wrong,stating 802.11g as using OFDM.So which is it guys n gals?One answer must be correct or is it a case of 802.11g being capable of using both OFDM & DSSS depending on which data transmission speed is being implemented?

 

Here you go Mal...

http://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum

You're going to ace this exam by the way.

J.

 

Nice one juice,much appreciated.So the answer is in fact BOTH,depending on implementation.Tonights little journey will take me through cables,cable stats,cable colours,cable connectors,cable testers,cable cables and cabling.

 

Hey, Mal.

Yes it's both - but not on the exam.

Just a heads up mate.

Gunning for ya!

J.

 

Now this is a real mindbender,the dreaded cables and all those damned stats that we're going to drill into our heads.Keep a cool head and join me as I start headbutting the table,scratch my head furiously and attempt to lay the facts down.No apologies for mistakes on this one,but again feel more than welcome to correct me..

All Things Cable!

I know you're going to flame me for this but I'm going to add the 10Base2 AND 10Base5 stats in here.Before you ask,yes I do know these two standards are now dropped from the N+ 2005 objectives but I spent a lot of time learning this stuff during my A+ and feel that knowing about legacy technology makes for competence,confidence and completeness,so there.


10Base5

Known as 'THICK ETHERNET' or 'THICKNET',it is the original flavour 50 OHM cable that gave us the BUS physical topology.The cable is rated as RG8.Thicknet requires an external transciever and an AUI port on the PC for connection.It is capable of handling 10Mbps at a maximum segment length of 500 metres.Thicknet can accomodate 100 nodes and demands each node be 2.5 metres apart.It transmits using BASEBAND technology.

10Base2

Known as 'THIN ETHERNET' or 'THINNET',it too runs over a 50 OHM cable and is used on a physical BUS network.It is rated as RG58.Thinnet uses an internal transciever (within the NIC) and 'T' connectors to establish a maximum segment length of 185 metres and can host up to 30 nodes.Nodes can be as close as 0.5 metres apart.It uses BASEBAND technology and connects via a BNC connector

Both Thicknet and Thinnet segments MUST be terminated at BOTH ends or the network will fail.

Thicknet and thinnet are now rarley seen in the real world environments that you and I might come across,but they are most definately not extinct,yet.Hence my insistence in having a little knowledge about them.

Thinnet is cheaper than Thicknet and is easier to work with.

10BaseT

This standard runs over UTP/STP cables,utilising 2 pairs for transmitting and recieving data.It connects via an RJ45 connector and has a maximum single segment length of 100 metres.It attains speeds of 10 Mbps and uses BASEBAND signalling.It requires a minimum of CAT3 cabling to implement.It can host a maximum of 1024 nodes on a STAR/BUS topology.

10BaseFL

I'd grant this standard the 'entry level' fibre tag really,since FL means Fibre Link over a 10Mbps fibre cable.Ranges are varied depending on what study material you use (and I use more than one,each with a slightly different set of stats!!!),I stick to the 500m up to 2000m explanation.It enables ethernet over fibre optic to your PC.It exclusively uses the ST connector.

100BaseT

100 Mbps running over UTP/STP with a minimum CAT5 rating to a maximum length of 100 metres.It will support up to 1024 nodes using BASEBAND signalling and implements the RJ45 connector.

100BaseFX

This standard runs up to 100 Mbps to a distance of 2000 metres over multimode fibre (MMF) using BASEBAND signalling.It uses the SC connector.

100BaseT4

This is not so common but still about I'd wager!It uses a CAT3 UTP/STP cable and ALL EIGHT wires to achieve 100Mbps,thats all I know about T4,sorry.

1000BaseT

Our first outing into Gigabit ethernet,now we're really rolling!This standard requires a MINIMUM of CAT5e cabling.It is capable of a blazing 1000Mbps and a maximum segment length of 100 metres.It uses BASEBAND signalling and utilises the RJ45 connector

1000BaseSX

This fibre optic standard is capable of 1000Mbps over a range of about 550 metres,running through MMF cable and connecting using SC connectors.

1000BaseCX

Not a common standard,or so I'm told,it runs gigabit ethernet over a maximum 25 metre length of STP,ideal for server racks and the like.It uses a connector called an HSSDC,which I have no intention of deciphering,just know what a HSSDC is methinks.

1000BaseLX

This standard runs up to 1000Mbps and can use BOTH multimode AND singlemode (MMF and SMF) fibre cabling at distances between 500 metres and 2000 metres and can support both the SC and ST connectors.

10GbaseSR

Ok poeple,hold on to your panties coz we're about to hit lightspeed!!!!well,10 gigabits per second,but thats fast,really fast.This standard is capable of 10000Mbps over MMF to a maximum length of 300 metres.The buzzword here is nanometres and this is the diameter of the optic cable,which in this case is 850nm.

10GbaseLR

10000Mbps over SMF to a length of 10000 metres,thats 10KM to you and me,count it-----TEN KILOMETRES!It runs over fibre with a diameter of 1310nm.

10GbaseER

Whooooooos ya daddy?This standard is,by far!This standard shunts 10000Mbps over SMF to a staggering distance of 40000 metres,I'll say that again...40000 metres....40KM in real money.It uses a cable with a diameter of 1550nm.

FDDI

Token ring over fibre,thats exactly what FDDI (fiddy) is.Two counter-rotating rings,each capable of 100 Mbps and providing excellent redundancy in the process.If one ring fails the other can still keep the network running,cool or what?FDDI can run at lengths of up to 10000 metres and uses a MIC connector.


And I'm done,seriously I am,this topic takes me endless attempts to get right,editing my mistakes (and I bet there are still a few) and remembering all the standards,acronyms,distances,cable types,connectors blah blah blah,but I DID warn you at the start of this one didn't I?I hope this stuff is accurate,since this is how I've learnt it so far and I urge you to correct any mistakes.If you're reading this and feeling bewildered then feel comfortable in the fact that so am I!I've heard from our N+ community that this stuff is usually a great points earner on the exam and should be learnt thoroughly,but don't take my advise on this one,ask someone who's done it..


Time now for a well earned cup of rosie and I'll ponder over the next exciting instalment,once I've decided what that is of course...

 

No mension of BNC

Great stuff mate!

 

Thanks for that,I've edited the post and added BNC as the connector.

I'm going to dedicate a short post to connectors,after all they are the one thing that we will all handle sooner or later and a little knowledge will go a long way.See you's all tonight then..

 

Really excellent stuff Mal.

Have you thought of publishing it as 'Mal's Network+ Exam Cram'?

Good on ya

J.