Archive for classful

WANs II: A New “Lease” on Life!

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 20, 2012 by jjrinehart

Private/Leased Line Network

One of the sayings I tend to use when explaining network concepts is a twist on the opening words of the book of Genesis, “In the beginning was the mainframe…” because so many things we take for granted today started with that piece of technology.  In terms of WAN connectivity, the original form of connectivity took place across specialized telephone lines that carried data rather than voice conversations.  Because the end-user/customer paid the IXC for the exclusive use of the line, they were referred to as private lines or point-to-point leased lines.  As mainframes were replaced by personal computers (and networks) these lines connected to devices that convert bits of data to electrical impulses that can be transmitted across these lines for long distances.  The technical term for this device is a Channel Service Unit/Data Service Unit, or CSU/DSU, which used to be an external device but are now integrated on interface modules on routers.  This is actually a good time to introduce two more terms, namely DCE and DTE.  A Data Terminal Equipment (DTE) device is the terminal or end-point sending and receiving information, usually a computer or router.  DCE devices, on the other hand, perform the conversion between raw data and the format needed for transmission, such as a CSU/DSU or modem.  The acronym stands for either Data Communications Equipment or Data Circuit-Terminating Equipment, depending on the publisher.  Cisco prefers the latter term as a general rule.

Private lines utilize electrical circuits to create the pathway between locations, as illustrated in the diagram above; this is in contrast to packet switched networks (which we will deal with later).  Depending on the part of the world you live in, there will be differing names and capacities, such as T1 (1.544 Mbps) or E1 (2.048 mbps), and even higher speeds.  These are typically copper connections, with very high speeds delivered on fiber optic connections (OC-X).  In North America these are the T1/DS1 and T3/DS3 standards, while the rest of the world utilizes the E1/E3 standards.  These lines are charged by mileage and often very expensive as a result, although very secure since a customer has private and full-time use of the connection, although if it ever sits idle that ends up being wasted bandwidth.  The structure of the framing and line coding ca be rather complex, and something you should definitely be familiar with as you pursue your certification studies.

Next time we will look at packet-switched WANs! Don’t you just love this stuff?

– Joe

WANs I: LANs, MAN’s and WAN’s Oh My!

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 12, 2012 by jjrinehart

A Wide Area Network

I started my career in the networking industry at a small Internet Service Provider in Seattle, Washington in the United States (this was in the late 1990’s).  At that time, most end-users accessed the Internet using dial-up connectivity, and modem speeds topped out at about 28 Kbps (yes, it sounds like ancient history).  From there I went to work for AT&T, where I spent the next five or so years assisting Fortune 500 businesses connecting multiple locations together; needless to say, I spent a lot less time dealing with Wide Area Networks than Local Area Networks.

Although I have since mastered LAN’s, I still have a great fondness for WANs and enjoy building labs that simulate them. LANs are suspiciously easy to recognize, first because they use Ethernet switches, but also because they occupy a fairly localized geographical area (hence the term LAN).  Wide area networks are also easy to recognize, as they almost universally depend on large telecommunications providers (AT&T, Verizon, British Telecom, etc.) and use an entirely different set of connections to provide services with.  A term that can be confusing, however, is that of a MAN or Metropolitan Area Network, and as such, needs some clarification.

The simplest way to differentiate MANs from WANs is to look at geography once again.  LANs connect computing devices with a floor, building, or campus, but no further than that.  WANs include networks that tie together sites across significant distances, such as nationally or internationally.  MANs are networks that lie within a smaller, more specific region; in a sense all MANs are WANs but not all WANs are MANS.  Think of the concepts like squares and rectangles; all squares are rectangles but not all rectangles are squares (using the classic shape definitions).

The more technical way of explaining Metropolitan Area Networks involves a little bit of US telecommunications history, specifically when AT&T was broken up in the 1980’s.  In the new system following this “divestiture”, new regions were created within states called local Access Transport Areas, or LATAs.  Local phone companies (e.g., Bell Atlantic, Pacific Bell, etc.operated in these LATAs, and Inter-Exchange Carriers (IXC’s) created connectivity between these areas; each had to operate separately.  In this arrangement, a MAN would be between locations within a LATA, while a WAN would be between them.  Usually this would encompass a city and its outlying suburbs and such, hence the term “metropolitan.”  With regulatory changes, these distinctions are not nearly as relevant, which explains why the term MAN is far less frequently used.

Next time we will look at some of the types of WANs that exist today!

– Joe

EIGRP III: Choosing the Winner of the Miss EIGRP Pageant!

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 8, 2012 by jjrinehart

The Winner! (Feasible Successor)

Even though EIGRP is a simpler protocol in many ways, there are still important concepts to understand, one of which how it chooses routes.  As we talked about before, the lowest metric (constrained bandwidth + cumulative delay) wins the contest for the best route, without the somewhat confusing exceptions that exist in OSPF.  However, because there are elements of Distance Vector routing in this hybrid protocol, there inevitably have to be some kind of loop prevention mechanisms.

EIGRP uses two forms of the calculated metric to a network to select the best loop-free route, and at first glance they might sound identical.  The first value is called the Feasible Distance (FD), which is the complete metric from the router to the destination network.  The second value is called the Reported Distance (RD), which is the metric from the point of view of the next-hop router.  If a path is loop-free, the value of the RD will be less than the FD, expressed mathematically as RD < FD.  Paths that could cause loops will have opposite values, which will make EIGRP discard it.  That may sound simple enough, but as you have probably heard on infomercials, “But wait, there’s MORE“!

Routes that meet the RD < FD test (called the Feasible Condition) are held in the EIGRP Topology Table and the best route gets installed in the IP Routing Table.  This best route is called the Successor, or Successor Route in EIGRP, and if other equal-cost routes exist they are also flagged as successors and installed in the routing table.  One of the truly amazing features of EIGRP, however, has to do with backup routes.  In just about every other protocol, if the primary route fails, the entire convergence process for that failed route starts over to select a new one.  In EIGRP, though, another loop-free path is held in reserve for immediate use if the successor fails; this route is called the Feasible Successor.

If you have ever watched any type of beauty pageant (Miss America, Miss Universe, etc.) then you have actually seen this in action!  After all of the contestants have proceeded through the judging process, one of them is crowned the winner (Successor)!  However, the next contest in line, usually called the first runner-up (Feasible Successor) automatically becomes the winner if anything happens to the original victor–no new pageant is required!

That covers the basic principles of IP routing, next time we will start considering Wide Area Networks!

– Joe

EIGRP II: Metric Calculation with a GPS…

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 6, 2012 by jjrinehart

GPS via Google Maps

Unlike most of you, I can get lost in my own backyard; imagine how misdirected I can get when I am actually driving!  My beautiful wife Brenda (a feisty little redhead) is a walking, talking, nearly-always-right human GPS, which is wonderful except when I am driving alone and trying to get somewhere.  Fortunately, in our world of GPS devices and smartphones (complete with Google Maps), I have some recourse for not getting lost.  Even so, these devices are not foolproof, as they once told me that a hotel was in the middle of the Potomac River in Washington DC!

These handy little devices are great because they rely on large databases which contain information on mileage, geography, construction, road conditions, etc., when trying to help get you from one place to another.  Just as GPS devices use multiple criteria for recommended a route of travel, so EIGRP relies on several different elements in calculating metric for destination networks.  This is remarkably different from every other interior routing protocol, which typically relies on a single element for its metric.  Here is a breakdown of the five elements of the EIGRP metric:

1. Bandwidth: At first glance you might think that this is identical to the OSPF cost concept, but there are a couple of important differences.  While bandwidth does create a cost-like factor (the higher the better), in EIGRP this cost is not cumulative.  Instead, it is based on the lowest bandwidth along the path to a destination network.  For example, if one path has all 100 Mbps links and another has 100 Mbps links with one 10 Mbps links, the first path will be preferred because the smallest (called constrained) bandwidth is 10 Mbps.  This might sound less ideal than a cumulative cost until you think of how backed up a highway gets when narrowed down to one or two lanes!

2. Delay: Unlike bandwidth, this factor is cumulative along the entire path.  The greater the delay, the less desirable the route is because delay is caused by lower bandwidth and/or congestion.  Why choose a 4 lane superhighway if the traffic is crawling along at a very slow speed?

3. Reliability: As the name implies, this measures how reliable the route is (0-255)

4. Load: How loaded or saturated the route is (0-255)

5. MTU: The IPv4 Maximum Transmission Unit size.

Keep in mind that only bandwidth and delay are enabled by default for metric calculation, and each element is called a K-Value.  Always make sure the K-Values match between neighbors or a relationship will never form.

Next time we will key in on route selection in EIGRP…

– Joe

EIGRP I: The “Borg” of IP Networking

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 3, 2012 by jjrinehart

Resistance is Futile!

While not completely universal, there often seems to be a natural affinity between networking geeks and the Star Trek science fiction franchise.  While I grew up watching reruns of the original series, I took a liking to the characters of the “next generation” cast (for you purists, TOS and TNG respectively), and found the Borg to be the most compelling super villains ever!  For those unfamiliar with these mechanical zombies, they are mechanically augmented humanoids all linked by technology to a central mind, being partly biological and partly machine.  Think of Darth Vader as “Borg Lite!”

The complexity of Borg characters is that they are not completely humanoid, though possessing certain biological characteristics, nor completely mechanical, though having aspects of that as well.  In essence, they represent a hybrid of the two, and it is worth noting that the term used for this type of being generally is a cyborg.  In other words, they are not one or the other, but a blend of both.  This is precisely the situation in which EIGRP finds itself, having characteristics of Distance Vector Protocols, as well as features of Link-State Protocols.

When you read white papers, books, and documentation on EIGRP, you will notice this type of duality present in characteristics of EIGRP.  For example, like LS protocols, EIGRP build formal neighbor relationships and tracks the state of those relationships.  Conversely, this protocol also uses the familiar DV loop prevention mechanisms such as split-horizon and hold-down states.  When you peruse the literature on EIGRP you will typically hear the word hybrid to reflect the nature of operations, although I have seem references to balanced-hybrid and advanced distance-vector as well.

A few similarities exist between EIGRP and OSPF, beyond neighbor relationships alone.  Although not a Link-State Database, EIGRP does build its own table of subnets called the Topology Database (more on this in another entry).  It also chooses a lowest-cost route to a destination subnet, but the criteria are entirely different from OSPF.

The differences between the protocols are more numerous that the similarities.  First, OSPF is based on an open standard, while EIGRP is Cisco proprietary.  In short, if you have non-Cisco devices in your network, you either have to do some form of redistribution (sharing routes between protocols) or you have to use OSPF or another standardized protocol.  Another significant difference is that EIGRP is not formal—no areas, DR’s/BDR’s, and so forth; no hierarchy exists, which certainly makes it simpler in many respects.

Next time we will dig into the EIGRP metric and route selection process.

– Joe

OSPF V: “You Have Chosen…Wisely” Path Selection Process

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 1, 2012 by jjrinehart

You Have Chosen…Wisely

In my opinion, the best movie in the Indiana Jones film franchise was Indiana Jones and the Last Crusade; aside from the pure enjoyment of the action scenes, the film gave great attention between “Indie” and his father.  One of the most-quoted lines from this movie, in pop culture at least, was the statement by the night (pictured above), “You have chosen…wisely.”  The whole concept of making the best possible choice ties in particularly well with OSPF, which puts it head-and-shoulders above Distance Vector routing protocols, since it truly can choose…wisely!  If you recall our earlier discussion about Link-State routing protocols, instead of depending on “mileage” (how far away something is”, the basis for route selection is cost, related to bandwidth.
OSPF uses cost in a cumulative manner–meaning that all of the costs of the links to a destination network are added up together.  If you have ever used a GPS for travel, this makes perfect sense, since the device would recommend the eight-lane Interstate highway (greater “bandwidth”) over the two-lane country road, regardless of the distance.  Ironically, you can even specify the route to avoid toll roads (a different spin on cost), to choose the best way to go.  If you think about it, this makes perfect sense, since you can go faster and have fewer stops on the bigger road, especially if you are tracking the route of travel from end-to-end.  In networking terms, then, OSPF devices will choose a 1.544 T1 link over the vastly inferior 56K link in choosing the best route.  What could be simpler?
Remember how strict and rule-oriented OSPF is?  Well, this applies to route preferences as well, meaning that there are additional selection criteria that will override the cost directive.  This fits into the hierarchy of OSPF areas, and creates the following list of route selection preferences:
1.  Intra-Area: Always choose the path within the area first.
2. Inter-Area: If no routes to the destination exist within the area, choose a path to another area but within the OSPF domain.
3. External Type 1: If no routes to the destination exist within the OSPF domain, choose an E1 route (remember that E1 routes count the cost to the external router in the metric)
4. External Type 2: If no E1 routes exist to the destination, choose an E2 route (remember that E2 routes do not count the cost to the external router in the metric, and redistributed routes are E2 by default)
As you can see, this can substantially change the route selection process.  Next time we will look at EIGRP, which is vastly simpler.
– Joe

OSPF IV: There is No “I” in Team: More about DR’s

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 30, 2012 by jjrinehart

DR vs. No DR

One of the unique features of OSPF concerns neighbor relationships across multiaccess networks, such as Ethernet LANs and certain types of WAN’s such as Frame-Relay and ATM (no, not the cash machines at banks).  Remember, neighbor relationships form between connected neighbors across links, so consider the diagrams above to get an idea of what this looks like. Here we have listed six routers, and the math required to calculate the number of relationships is N(N-1)/2, where N is the number of devices.  Solving for 6 in this case creates the values of 6 * 5, yielding 30, and dividing that by 2 results in 15, which is a lot between so few devices, and staggering between many.  To simplify this, OSPF has a single peer on multiaccess networks called the Designated Router, or DR.  Note in the diagram how the 15 relationships is reduced to 5 using a DR, and in reality there are 5 more with a secondary DR, called the BDR.  In keeping with the highly regimented structure of the protocol, all messages, updates, and so forth take place between peers and the DR, and NOT with one another.  The purpose of the BDR is to take over if anything happens to the DR.

As is the case with many other network protocols, there is an election process to determine the DR and BDR roles on the multiaccess networks, and seldom is this optimal.  I have personally had to deal with suboptimal DR selection in networks and labs I have worked on, especially when dealing with redistribution (the process of sharing routes between various routing sources).  Each router on the OSPF multiaccess network has a default priority of 1, which usually results in a tie in the election process, and the highest numerical Router-ID wins if that is the case.  To set the priority manually, use the ip ospf priority <0-255>  command on the interface, understanding that the higher the priority, the better (I routinely use 200 for the DR and 190 for the BDR).  To remove a router from the election process, just specify zero (0) as the priority, which is useful in hub-and-spoke topologies such as Frame-Relay (having a spoke router as the DR or BDR is not helpful at all).

Let me share a quick word on WAN topologies in OSPF because they can drive you crazy at times.  Frame-Relay and ATM do not forward broadcasts naturally, and special configuration is required is you want that functionality.  If you have to work with non-broadcast links, use the ospf neighbor command under the OSPF process.

Next time, we will wrap up our OSPF discussion by looking at route selection.

– Joe

OSPF III: DR Seeking BDR fot LTR (Neighbor Relationships, cont’d)

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 27, 2012 by jjrinehart

Is He/She a Keeper?

Few things rattle us more than the pursuit of that “special someone” in our lives, regardless of our respective cultural backgrounds.  In some cultures, marriages are arranged (often numerous years ahead of time), while others leave the process up to the individual, often found in Western cultures through the ritual of dating.  Regardless of how the process develops, how the relationship develops follows a relatively common pattern.  In cultures where dating is the accepted path, the journey begins with the often-awkward first date, when the person across the table is basically a stranger.  If things go well, conversations pass back and forth, common interests and viewpoints are discovered, and a relationship begins to form.  As things progress over time, greater trust is established, deeper conversations ensure, and at some point the two individuals become a couple in a more formal sense.  What does this have to do with OSPF?  Actually, everything!

OSPF relationships follow the same pattern just described in the whole dating/mating ritual in the previous paragraph.  They start out as strangers, with no trust, and not sharing vital, if any, information about what they know.  Using Hello Messages, the routers start the process of conversing, and over time the relationship goes from “perfect strangers” to “fully adjacent” in which Link-State information is exchanged.  Nothing about the process is instant, it takes a period of time, although the procedure is fairly rapid by human standards.  There are seven stages of states of the relationship building process, as follows:
  1. Down: Not aware of one another
  2. Init: Initializing, hello packets sent
  3. Two-Way: Neighbor sees its own Router-Id in the hello packet
  4. Exstart: Adjacency/Relationship formed, Database Description Packets created
  5. Exchange: Database Description Packets sent to neighbor
  6. Loading: Slave device sends Link State Requests and received Link State Advertisements
  7. Full: LSDB identical, and neighbors ready to forward traffic

The next point is important because it can help you figure out when something is wrong.  Occasionally, neighbor relationships stop before reaching the Full state, indicating that some sort of problem is preventing full adjacencies from forming.  You can check the current state of any OSPF neighbor by executing the show ip ospf neighbor command from the Command Line Interface, which will list the Router-ID, state, etc., of the neighboring device.  Most commonly, this will happen at the Init, Two-Way, or Exstart stages, and requires some troubleshooting (remember that OSPF is very picky, so there are several causes that you can investigate further).

Next time we will look into the Designated Router concept.

– Joe

OSPF Part II: Wouldn’t You Like to be My Neighbor?

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 25, 2012 by jjrinehart

In my opinion, one of the most misunderstood words in modern society is the word neighbor, particularly in the United States.  The term is extraordinarily broad, and can refer to a fellow resident of a housing development or apartment, someone in the same city or county, or even country.  When you consider how loosely the word is used today, you can easily get confused when you start talking about neighbor relationships between routers, because the definition is stricter.  Incidentally, when I grew up in the 1970’s in Pennsylvania, the word neighbor meant the people living on either side of you, or across the street.  Since I don’t expect everyone that reads my writing to have that same set of experiences (either in terms of chronology, geography, or culture), let me invoke a more familiar example: the characters of Tim Taylor and Wilson on the television show Home Improvement.  Wilson and Tim lived right next door to one another, and shared both a fence and property line, meaning that their yards were literally connected.  In just about every episode, these two individuals carried on conversations, usualy with Tim asking Wilson for insight and advice, and usually heeding what was said.

Let me point out how their relationship worked as neighbors.  First, they were connected by a common property line, so there was nothing between them other than the fence, which allowed them to interact easily.  Second, they had an actual relationship, which involved a level of trust; in other words, they were not strangers.  Finally, they carried on conversations, in essence, exchanging information, which resulted in at least one of them changing something they had been thinking or doing previously.  Now let’s apply that to how OSPF defines neighbor relationships.  Neighbors have to be directly connected, without another network separating them; a link of some kind connects them (just like the property line).  Second, these relationships are not casual, the OSPF neighbor routers have a very formal relationship and know and trust one another.  Finally, they exchange information, specifically, Link State Advertisements, the pieces of data that allow for the Link State Database to be duplicated on the other device.  This is much more structured than RIP which just accepted whatever it hear as in fact being true and accurate.  In fact, OSPF routers track the state and/or availability of the neighbor device, sending messages at regular intervals called Hello Messages (more on that later), which transmit data as well as act as a keepalive mechanism.

Next time we will step through how OSPF routers build these relationships.

– Joe

OSPF Part I: The Drill Sergeant of Networking!

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 22, 2012 by jjrinehart
I have never been in any branch of the military, but I have known a number of brave men and women that have served.  For those who may not know how the military induction process works, the first step is known as Basic Training, or more affectionately, hell on earth!  This phase involves heavy physical activity, intense discipline,and hours of a drill sergeant (see graphic above) barking orders in a fierce, shrill tone that makes fire freeze!  By now you might be wondering what on earth any of this has to do with networking, which is a fair question.  Enter the Open Shortest Path First protocol, abbreviated OSPF.
Everything in OSPF is about rules, and ensures that routing takes places in an even and consistent manner across the entire autonomous system (network under a common administration).  There are rules about design, rules about information exchange, rules about the routing hierarchy, and about everything else you can think of, so to speak.  Needless to say, OSPF is also highly structured, and that is part of the appeal of this best-known link-state routing protocol.
In keeping with the military analogy, one of the first things to understand about OSPF is the “chain of command” or hierarchy.  Just as there are platoons, squads, and units of soldiers in any army, there are specific groups of devices in this protocol, which are called areas.  With a small network, you can get away with a single area and keeps things relatively simple.  In larger networks, multiple areas are a given even if just for scalability (a term meaning the ability to grow in a measured fashion), but there are often additional, design-related, reasons as well.  Areas are identified using decimal numbering, such as 0, 1, 2, and so on, although you can also use dotted decimal numbering such as 10.1.4.13; in my own experience I have only seen the digit form of numbering.
Getting back to the “rules”, an important thing to understand is that all areas must connect to a special area, called the Backbone Area, or Area 0.  In other words, any traffic leaving one area and destined for another must cross Area 0.  If you have just one area in the entire network, then you can number it just about any way you like, but in any multi-area OSPF network you simply must have an Area 0, through which traffic passes.  There is an exception process using a special connection called a Virtual-Link (which creates a direct connection to Area 0 by one that is separated by another area), but that is beyond the scope of the CCNA.
Next time we will look at formal neighbor relationships!
– Joe