Friday, February 25, 2011


(Note that this is not an official Basic Draft article. It does not meet Basic Draft standards and will not be listed. If you've arrived through a search engine, please use the search function (top-left) to find a more suitable article).
I know I shouldn't be doing this but I'll be doing it anyway. I'm Basic Drafting NETINFRA. This is so far the most badly polished subject I've ever encountered. I am already having a half time agreeing with certain things taught, let alone Basic Draft about it, but I'm doing it anyway, because I wish to get this subject over and done with.

It's weird that Basic Draft is now becoming a verb.

Alright let's all begin with IP Addressing. I'm sure I don't need this and neither should anyone else, but let's go! (I'll be brief)

An IP address is a 32-bit value that is represented in 4 dotted-octets (that is to say, 4 groups of numbers that are 255 each separated by dots). An example of an IP address is... Uh...

Each IP address is combined with a subnet mask. A subnet mask is combined with the IP address to determine these two things:
1) What is the network or (in the case of classless addressing - subnet) address?
2) What type of address is it? (Network? Host? Broadcast?)

IP Addressing is used in Layer 3. Like anything else in the OSI layer, a higher layer abstracts the lower layers. In other words, it is independent of the actual physical device and standards being used in Layer 2 and 1.

IP Addressing is a logical and hierarchical addressing standard. Like hierarchical folder structures, IP addresses are arranged as a tree.

Here's a breakdown of the operations that you can perform on an IP Address and Subnet Mask:
Subnet Address = IP & Subnet Mask
Broadcast Address = Subnet Address + Increment - 1
Increment = Value of lowest set bit in Subnet Mask
Number of Hosts in that Subnet = Increment - 2
Number of Subnets = 2^(Classful CIDR - CIDR) - 2
Range of Host = (Subnet Address + 1) to (Subnet Address + Increment - 2)

Okay I think this is unnecessary so I'll skip faster from now on.

The class of the address depends on the highest order bits of the IP address:
Class A = 0 = 1-126 (127 is reserved for Localhost)
Class B = 10 = 128 - 191
Class C = 110 = 192 - 223
Class D = 1110 = 224 - 239
Class E = 1111 = 240 - 254

These are the various classful subnet masks and their CIDR:
Class A = = /8
Class B = = /16
Class C = = /24

CIDR is simply the number of contiguous 1's in a subnet mask.

Each network/subnet represents a broadcast domain (the area of extent a broadcast can travel). Everything in a network/subnet would receive a broadcast from that subnet.

Among other things, networks can be broken into subnets to increase broadcast domains. Subnets also allow an organization to be better structured, e.g. can be broken into 4 subnets (or traditionally 2) of 192.168.1.x/26. Each of these subnets can be assigned to a department.

Okay I'm too lazy to talk about subnetting anymore. I'll just go through one worked example and that's it.

Suppose that I have a network and I have 50 subnets that I want to assign. Each subnet is expected to have only up to 500 hosts. Ensure minimal borrowing of subnet bits.

The minimum number of borrowed bits is roundUp(log2(50+2)), which is about 6.

6 subnet bits would give me (2^6)-2 = 62 subnets.

The default mask is /16, so the new mask is /22. The increment of /22 is 4, so subnet 5 would be:

172.16.(5*4).0/22 =

The host range would be to 172.16.(20+4-1).254 =

Each subnet would have 2^(32-22)-2 = 1022 hosts, so it satisfies the 500 hosts requirement.

Okay we don't need any more examples, let's move on to WAN and Routers.

We first classify network devices by DTE and DCE. As taken from Article CCNA 9:

DTE (Data Terminal Equipment):

DCE (Data Communications Equipment):
-Modem or CSU/DSU

The different cable implementations can be found in CCNA 9.

Here's a quick summary:
DTE-to-DTE = Cross-over
DCE-to-DCE = Cross-over
DTE-to-DCE = Straight-through
Console Port = Rollover

LAN uses Cat5e UTP/STP while WAN uses Serial.

Here are the various WAN devices if the examinations ask for it:
-WAN Switch (such as for Frame Relay)
-Modem or CSU/DSU
-Communication Server (used for concentrating DIDO (dial-in/dial-out) connections, such as a RADIUS server with multiple modems)

As a NETFUND review, WANs operate over large geographical area, allows access through serial at lower speed, and provide full and part-time connections (always-on vs on-demand).

WANs are typically used to connect sites together. Exchange of information between these sites are done through WAN devices (listed above).

Routers are used to interconnect LANs. Typically routers also have Serial interfaces to connect to a WAN.

Unrelated, but stated in the text, the different roles a router in a OSPF domain can take are:
- Internal Routers
- Backbone Routers
- Area Border Routers
- Autonomous System Boundary Routers

This is definitely not tested because it is not supposed to be in the seminar in the first place.

If you want to learn about Link State protocols, visit CCNP BSCI 05.

WAN switches exist in a WAN cloud to provide switching at Layer 2 to provide transparent connectivity. They are operated by the ISP.

Modems (Modulator/Demodulator) are used for Digital to Analog conversion typically for transmission over a voice-grade line.

CSU/DSUs are used for connection to a Digital facility, such as a Frame Relay WAN switch, or a T1 leased line. The CSU (Channel Service Unit) is used for that connection, and the DSU (Data Service Unit) is used for diagnostic functions pertaining to the telecommunications line.

In a normal real-life WAN scenario, the Router is a DTE equipment, while the CSU/DSU is a DCE end.

WAN standards are scattered through OSI Layers 1 and 2.

The physical layer describes the WAN's Electrical, Mechanical, Operational and Functional aspects. Examples of physical layer WAN technologies are the various cable implementations and connection types. The DTE and DCE definitions are also WAN standards.

The DCE end of each serial connection sets the clock rate. The ISP's CSU/DSU is the demarcation point, where responsibilities of connectivity between the ISP and customer are separated.

If asked during the examination, these are the various Layer 1 WAN connection types:

Also, if asked, here are the world's main WAN standards definers:

The Layer 2 WAN standard describes encapsulation protocols. These protocols are the language spoken between two connected devices. An example of a LAN protocol is the Ethernet.

If tested, Layer 2 WAN protocols provide services for these connection types:
-Multi-Access Switched

(These three things are badly categorized anyway, since Multi-Access Switched is how a Point-to-Multipoint connection is implemented.)

The WAN Layer 2 encapsulation exists between the WAN interfaces of two connected routers.

If tested, here are the various WAN protocols:
-Frame Relay

Here are the various (very incomplete and misleading) definitions to memorize in case tested:

HDLC - Default Cisco Encapsulation, Proprietary, supports Point-to-Point and Point-to-Multipoint, and uses minimal overhead

Frame Relay - Uses High Quality digital switching facilities, requires error checking at Data Link layer, uses Simplified Framing with no error correction, is Connectionless, and Routing and Switching is performed at the Data Link Layer (This whole chunk makes no sense. Please memorize it)

PPP - Developed by IETF to replace SLIP, can support Analog circuits (such as for Dial-Up), provides Error Correction, able to encapsulate several routed protocols, can check for Link Quality during establishment, supports PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol).

(In my opinion, if that's the way the school is going to teach the three encapsulations, I rather they not teach it.)

A dedicated line is a single line that is established end-to-end that is dedicated to only two sites. Examples of dedicated line technologies are T1/E1/T3/E3, xDSL and SONET. It is a point-to-point implementation. PPP and HDLC can be used over this.

A circuit switched connection is a connection routed over an analogue circuit such as a voice-grade telephone service. PPP, POTS Dial-Up and ISDN can be used over this.

A packet switched connection is a connection routed over digital facilities, such as a Frame Relay cloud (consisting of L1 Frame Relay switches). Frame Relay encapsulation is used over this.

A cell switched connection is a packet-switched connection that requires fixed-size padded cells to be transmitted. Examples are ATM and SMDS.

The above categorization was taken from the Lecture slides (with some extra explanation put in by me). Notice that ISDN can also be dedicated, and POTS is not an encapsulation. The whole slide is confusing and groups unrelated categories together. Right now, just know what technology is related to what type (e.g. Dedicated/Circuit/Packet Switched).

Dig up my CCNA articles to learn how the encapsulations can be used.

Now let's talk about Routers. Routers are Layer 3 devices designed to interconnect networks of different types (LANs/WANs/MANs). A router provides routing of packets (not switching of packets as described in the slides) and determines the best path (i.e. Path Determination).

Router needs a DCE device to connect to WANs (either an external DCE device or a built-in one like a CSU/DSU card). In a LAN environment, we can simulate WAN connections between two routers using a Cross-over Serial Cable (or professionally referred to as a NULL Modem Serial Connection). It is made using a special back-to-back serial cable with a DCE and DTE end. The DCE end requires the "clock rate" command to be issued (i.e. It issues/provides the clocking signal).

For a piece of data to be successfully transmitted, it must have two types of addresses: Layer 2 and Layer 3 addresses. The Layer 2 addresses changes point-to-point, it determines which is the next routing node the frame goes to. The Layer 3 address remains the same through the whole transfer, as it determines the initiating and receiving end.

Examples of Layer 2 addresses are:
MAC, Frame Relay DLCI, and HDLC LLC.

Examples of Layer 3 addresses are:
IP, IPX, DecNET, AppleTalk

Data is encapsulated as shown:
|Layer 2|Layer 3|Layer 4|Data|Layer 2 Trailer|

In the Layer 2, there typically exists protocol-dependent fields like Source/Destination MAC and Dot1Q header. The trailer is used for error detection and correction. If data is encapsulated at Layer 2, it is referred to as a Frame.

In the Layer 3 encapsulation, we would have things like IP address and IP Precedence (in case of IP). If data is encapsulated only up to Layer 3, it is referred to as a Packet.

There of course is a Layer 4 which contains typically TCP or UDP information such as Source and Destination Ports.

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