ICND1 break down

WAN trouble-shooting commands

Posted on November 12, 2009. Filed under: CCNA, ICND1 break down |

content moved, check updated WAN Trouble-shooting on xyznetwork.blogspot.com

 

 

 

 

 

 

 

 

 

 

 

 

 

Let’s talk a little bit about IOS commands for WAN trouble-shooting.

To verify the physcial cable connection on the routers, “show controller serial 1”, where serial 1 is the serial port the cable attached to.

R1#show controller serial 1

For the sake of trouble-shooting, we may want to use command “show interface serial1” to gain more information about the interfaces. Sometimes we find that the physical interface is up and the line procol down, generally there could be two reasons

We forget to set the clock rate on DCE. The line protocal will be down after 30 seconds, because the DTE need to receive the clock rate to work correctly. To set the clock rate on the DCE, use command “clock rate 56000” to set the clock rate to, for instance, 56 kbps.

The encapsulation type mismatch on both ends of the connection.

For example, if we physically connected router R1 and R2, but set different encapsulation type on them.

R1#encapsulation ppp

R2#encapsulation hdlc

Then, we will see physical interface up and line prococal down by running command “show interface serial1” on both router R1 and R2. To resolve the problem, we issue command “encapsulation hdlc” on R1 or issue command “encapsulation ppp” on R2, so that the encapsulation type matches on both ends of the serial link.

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Routing Process Continued — Behind the PING

Posted on September 1, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |

content moved, check updated Routing Process Continued — Behind the “PING” on xyznetwork.blogspot.com

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ICND1 and ICND2 break down

Understanding the routing process is critical to pass your CCENT and CCNA exam, and it is also the foundamental of networks, so let’s make it crystal clear by looking at an example.

The modern Internet Ping command refers to a program was written by Mike Muuss in December, 1983. It sends a small packet of information containing an ICMP ECHO_REQUEST to a specified computer, which then sends an ECHO_REPLY packet in return. Let’s exam what happens after a ping command is issued.

10038[1]

Refer to the exhibit. The LAN contains 2 hosts, 2 hubs and 2 routers. With subnetmask 255.255.255.240 or /27, router R4 and R5 divide the LAN into 3 subnets — host A and R4’s 192.168.10.33/27 interface belongs to 192.168.10.32 subnet; R4’s 192.168.10.65/27 interface and R5’s 192.168.10.66/27 interface belongs to 192.168.10.64 subnet; R5’s 192.168.10.129/27 interface and host B belongs to 192.168.10.128 subnet.  Notice the layer 1 device hub have neither IP address nor ethernet address.

Now, host A pings host B, what happened exactly behind the magic “PING” command.

  • Step 1, command “ping 192.168.10.134” is issued in the console at host A.
  • ping program is envoked, which reads IP address 192.168.10.138 from user input, and hands it to Internet Control Message Protocol (ICMP).
  • ICMP then creates an echo request payload.
  • ICMP hands the payload to Internet Protocol (IP). IP then creates a packet with IP source address 192.168.10.34 and destination address 192.168.10.134. The Protocol field of the packet has value 0x01, which means ICMP.
  • After the packet creation, IP determines whether the destination IP address is on the local network or a remote network. The subnet mask stored on Host A 255.255.255.240 is bitwise AND to host A’s IP address 192.168.10.34 to determine that host A belongs to 192.168.10.32 subnet. With the same manner, IP determines that host B belongs to another subnet 192.168.10.128.
  • Since IP determines that this is a remote request, the packet needs to be sent to the default gateway so the packet can be routed to the remote network. The default gateway is stored in host A as 192.168.10.33 (either statically configured by the user or dynamically configured by DHCP).
  • Because hosts only communicate via hardware addresses on the local LAN, for this packet to be sent to the default gateway, the hardware address of the default gateway (router’s interface with IP address 192.168.10.33) must be known.
  • The ARP cache of the host is checked to see if the IP address of the default gateway has already been resolved to hardware address. If it has, the packet is then handed to the Data Link layer with the hardware destination address. Otherwise, an ARP broadcast (to IP address 192.168.10.63) is sent out onto the broadcast domain (subnet 192.168.10.32) to search for the hardware address of 192.168.10.33. The router responds to the request with hardware address of ethernet interface 192.168.10.33, and Host A caches this address.
  • IP hands the packet down to Data Link layer for framing. The Data Link layer frames the packet of information and includes the following in the header: the destination hardware address 9999.DADC.1234, the source hardware address BBBB.3333.5677, the Ether-Type field with 0x0800 (IP) in it, and the FCS field with the CRC result.
  • The frame is now handed down to the physical layer to be sent out over the network mdeium one bit at a time.
  • The router R4’s Ethernet interface with MAC address 9999.DADC.1234 receives the bits and builds a frame. The CRC is run, and FCS field is checked to make sure the answers match.
  • Once the CRC is found to be okay, the hardware destination address is checked. Since the router’s interface is a match, the packet is pulled from the frame and the Ether-Type field is checked to see what protocol at the Network layer the packet should be delivered to.
  • The protocol is determined to be IP, so it gets the packet. IP runs a CRC check on the IP header first and then checks the destination IP address. The destination address is 192.168.10.134, which donesn’t match any of the router R4’s interfaces. Therefore, the routing table is checked to see whether it has a route to 192.168.10.134. If there’s no entry found for 192.168.10.134, the packet will be discarded. If there’s an entry found for 192.168.10.134, (For example, command “show ip route” reveals an entry such as “S    192.168.10.134/24 [1/0] via 192.168.10.66“) the packet is ready to be sent out from interface 192.168.10.65, which directly connects to the next hop 192.168.10.66.
  • The router checks the ARP cache to determine whether the hardware address for 192.168.10.66 has already been resolved. If it has, the packet is then handed to the Data Link layer with the hardware destination address. Otherwise, an ARP broadcast (to IP address 192.168.10.95) is sent out onto the broadcast domain (subnet 192.168.10.64) to search for the hardware address of 192.168.10.66. The router R5 responds to the request with hardware address of ethernet interface 192.168.10.66, and Router R4 caches this address.
  • The hardware address and packet are handed to the Data Link layer. The Data Link layer builds a frame with the destination address (MAC address corresponding to 192.168.10.66, not shown in the exhibit) and source hardware address (MAC address corresponding to 192.168.10.65, not shown in the exhibit) and then puts IP in the Ether-Type field. A CRC is run on the frame and the result is placed in the FCS field.
  • The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time.
  • The destination Router R5 receives the frame, runs a CRC, checks the destination hardware address, and looks in the Ether-Type field to find out whom to hand the packet to.
  • The protocol is determined to be IP, so it gets the packet. IP runs a CRC check on the IP header first and then checks the destination IP address. The destination address is 192.168.10.134, which donesn’t match any of the router R5’s interfaces. Therefore, the routing table is checked to see whether it has a route to 192.168.10.134. If there’s no entry found for 192.168.10.134, the packet will be discarded. But router R5 is directly connected with Host B, there should be an entry in the routing table like “C    192.168.10.134/24 is directly connected, FastEthernet0“.  This means the packet is ready to be sent out from interface 192.168.10.129, which directly connects to the Host B 192.168.10.134. Notice hub don’t have IP address, it is just a multi-port signal repeater.
  • The router checks the ARP cache to determine whether the hardware address for 192.168.10.134 has already been resolved. If it has, the packet is then handed to the Data Link layer with the hardware destination address. Otherwise, an ARP broadcast is sent (to IP address 192.168.10.159) out onto the broadcast domain (subnet 192.168.10.128) to search for the hardware address of 192.168.10.134. The Host B responds to the request with hardware address DDDD.4444.1357, and Router R5 caches this address.
  • The hardware address and packet are handed to the Data Link layer. The Data Link layer builds a frame with the destination address DDDD.4444.1357 and source hardware address 5555.AAAA.6666 and then puts IP in the Ether-Type field. A CRC is run on the frame and the result is placed in the FCS field.
  • The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time.
  • The destination Host B receives the frame, runs a CRC, checks the destination hardware address, and looks in the Ether-Type field to find out whom to hand the packet to.
  • IP is the designated receiver, and after the packet is handed to IP at the Network layer, it checks the protocol field for further direction. IP finds instructions to give the payload to ICMP, and ICMP determines the packet to be an ICMP echo request.
  • ICMP creates an echo reply payload.
  • ICMP hands the payload to Internet Protocol (IP). IP then creates a packet with IP source address 192.168.10.134 and destination address 192.168.10.34. The Protocol field of the packet has value 0x01, which means ICMP.
  • After the packet creation, IP determines whether the destination IP address is on the local network or a remote network.
  • Since IP determines that this is a remote request, the packet needs to be sent to the default gateway so the packet can be routed to the remote network. The default gateway is stored in host B as 192.168.10.129 (either statically configured by the user or dynamically configured by DHCP).
  • The hardware address of 192.168.10.129 is found with ARP process, and the hardware address 5555.AAAA.6666 and packet are handed to the Data Link layer.
  • The Data Link layer builds a frame with the destination hardware address 5555.AAAA.6666 and source hardware address DDDD.4444.1357 and then puts IP in the Ether-Type field. A CRC is run on the frame and the result is placed in the FCS field.
  • The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time.
  • The destination router R5 receives the frame, runs a CRC, checks the destination hardware address, and looks in the Ether-Type field to find out whom to hand the packet to.
  • IP is the designated receiver, and after the packet is handed to IP at the Network layer, IP runs a CRC check on the IP header first and then checks the destination IP address. The destination address is 192.168.10.34, which donesn’t match any of the router R5’s interfaces. Therefore, the routing table is checked to see whether it has a route to 192.168.10.34. If there’s no entry found for 192.168.10.34, the packet will be discarded. If there’s an entry found for 192.168.10.34, (For example, command “show ip route” reveals an entry such as “S    192.168.10.34/24 [1/0] via 192.168.10.65“) the packet is ready to be sent out from interface 192.168.10.66, which directly connects to the next hop 192.168.10.65.
  • The router checks the ARP cache to determine whether the hardware address for 192.168.10.65 has already been resolved. If it has, the packet is then handed to the Data Link layer with the hardware destination address. Otherwise, an ARP broadcast is sent out onto the network to search for the hardware address of 192.168.10.65. The router R4 responds to the request with hardware address of ethernet interface 192.168.10.65, and Router R5 caches this address.
  • The hardware address and packet are handed to the Data Link layer. The Data Link layer builds a frame with the destination address (MAC address corresponding to 192.168.10.65, not shown in the exhibit) and source hardware address (MAC address corresponding to 192.168.10.66, not shown in the exhibit) and then puts IP in the Ether-Type field. A CRC is run on the frame and the result is placed in the FCS field.
  • The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time.
  • The destination Router R4 receives the frame, runs a CRC, checks the destination hardware address, and looks in the Ether-Type field to find out whom to hand the packet to.
  • The protocol is determined to be IP, so it gets the packet. IP runs a CRC check on the IP header first and then checks the destination IP address. The destination address is 192.168.10.34, which donesn’t match any of the router R4’s interfaces. Therefore, the routing table is checked to see whether it has a route to 192.168.10.34. If there’s no entry found for 192.168.10.34, the packet will be discarded. But router R4 is directly connected with Host A, there should be an entry in the routing table like “C    192.168.10.34/24 is directly connected, FastEthernet0“.  This means the packet is ready to be sent out from interface 192.168.10.33, which directly connects to the Host A 192.168.10.34. Notice hub don’t have IP address, it is just a multi-port signal repeater.
  • The router R4 get the hardware address for 192.168.10.34 with ARP process.
  • The hardware address and packet are handed to the Data Link layer. The Data Link layer builds a frame with the destination address BBBB.3333.5677 and source hardware address 9999.DADC.1234 and then puts IP in the Ether-Type field. A CRC is run on the frame and the result is placed in the FCS field.
  • The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time.
  • The destination Host A receives the frame, runs a CRC, checks the destination hardware address, and looks in the Ether-Type field to find out whom to hand the packet to.
  • IP is the designated receiver, and after the packet is handed to IP at the Network layer, it checks the protocol field for further direction. IP finds instructions to give the payload to ICMP, and ICMP determines the packet to be an ICMP echo reply.
  • ICMP acknowledges the ping program that it has received the reply, ping program then sends an exlamation point (!) to the user interface.
  • ICMP then attempts to send four more echo requests to the destination host.

We have walked through the ping process step by step in the above demonstration. All these steps are hidden behind a single command “ping 192.168.10.134”. As the packet traverses from router to router, layer 3 source and destination addresses do not change when the packet traverse, whereas layer 2 frame header and trailer are removed and replaced at every layer 3 device.

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Crosstalk

Posted on August 12, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |

content moved, check updated Crosstalk on xyznetwork.blogspot.com

  • Crosstalk is unwanted signals coupled between adjacent wire pairs. Since 1000BASE-T uses all four wire pairs, each pair is affected by crosstalk from the adjacent three pairs. Crosstalk is characterized in reference to the transmitter. At higher transmission frequencies, the crosstalk will increase, result in the destruction of more of the data signal.
  • Near-end crosstalk (NEXT) is crosstalk that appears at the output of a wire pair at the transmitter (near) end of the cable.
  • Far-end crosstalk (FEXT) is a measure of the unwanted signal coupling from a transmitter at the near-end into a neighboring pair measured at the far-end.
  • Equal level far-end crosstalk (ELFEXT) is a measure of the unwanted signal coupling from a transmitter at the near-end into a neighboring pair measured at the far-end relative to the received signal level measured on that same pair.
  • Power sum equal level far-end crosstalk (PSELFEXT) is a computation of the unwanted signal coupling from multiple transmitters at the near-end into a pair measured at the far-end relative to the received signal level on that same pair.

ICND1 and ICND2 break down

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ICND1 break down — TCP and UDP

Posted on June 30, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |

Site no longer maintained, updated post is here

The transport Layer, Layer 4, is residing between the application and network layers. TCP and UDP are the most important Transport Layer protocal. Their purpose is to identify the application from which the message was received and create segments to be passed down to the Internet layer. TCP protocol also provide two additional functions:

  • Flow Control: a mechanism that enables the communicating hosts to negotiate how much data is transmitted each time with sliding window.
  • Reliability service: a mechanism that guarantee the delivery of each packet by using sequence numbers and acknowledgments.

UDP

UDP is a connectionless and unacknowledged protocol. UDP only transmitts messages with “best effort”, it does not check the delivery for segments. UDP depends on upper-layer protocols for reliability. Note that broadcast and unicast messages are carried by UDP. The protocols that use UDP include TFTP, SNMP, NFS and DNS.

The following is UPD segment structue.

bits 0 – 15 16 – 31
0 Source Port Destination Port
32 Length Checksum
64  Data

The UDP header consists of only 4 fields. The use of two of those is optional (pink background in table). The following list is a brief explaination of the UDP header fields, here is the detailed explaination.

  • Source port — ID of the calling port.
  • Destination port — ID of the called port.
  • Length  — Length of UDP header and UDP data.
  • Checksum  — Calculated checksum of the header and data fields.

TCP

TCP is a connection-oriented, reliable protocol. TCP must establish a connection (a virtual circuit) between both ends user applications before transfer of information can begin. The services provided by TCP is running in the host computers at either end of a connection, not in the network. Therefore, TCP is a protocol for managing end-to-end connections. TCP is responsible for

  1. breaking a message passed down from the session layer into multiple segments,
  2. attaching a sequence number to each segment,
  3. passing the segments down to the network layer at the source station,
  4. veryfying each segment passed up from the network layer at the destination station,
  5. reassembling received segments into a message,
  6. and finally passing the message up to the session layer.

For a connection to be established, the two end stations must synchronize on each other’s initial TCP sequence numbers. This initial exchange ensures that lost data can be recovered.

The following steps are followed in this initial synchronization:

  1. A –> B SYN – My sequence number is X
  2. A <– B ACK – Your sequence number is X -1; expect X + 1 next
  3. A <– B SYN – My sequence number is Y
  4. A –> B ACK – Your sequence number is Y -1; expect Y + 1 next

Because step 2 and 3 are combined into one message, it is called a three-way handshake.  The following diagram might better illustrate this process.

three_way

TCP will return an acknowledgment to the sender upon receipt of one or more segments. There is a field called Acknowledgment number in the TCP segment. The receiving TCP use this field to tell the sending TCP which segment to receiving TCP expecting to receive next.

In case the sender transmitting too fast, the receiver will use a TCP flow control mechanism.

  1. Drop the segments: Failed acknowledgments alert the sender to slow down or stop sending.
  2. Set a smaller window size: Each TCP acknowledgement contains a field called window Size, which specifies the number of bytes that the receiving TCP is currently prepared to receive. Setting the window size to a smaller value allows less data to be processed in the future. More specific, the widow size is the number of data segments the sender is allowed to send before getting acknoledgment from the receiver. A smaller window size means the sending TCP has to wait for more acknowledgement in order to send the same amount of data, as a result, the time cause by these extra acknowledgment slows down the data transmission process.

windowing

TCP segment structure

 
Bit offset 0–3 4–7 8–15 16–31
0 Source port Destination port
32 Sequence number
64 Acknowledgment number
96 Data offset Reserved CWR ECE URG ACK PSH RST SYN FIN Window Size
128 Checksum Urgent pointer
160 Options (optional)
160/192+  Data

The TCP header consists of 11 fields, of which only 10 are required. The eleventh field is optional (pink background in table) and is aptly named “options”. The following list is a brief explaination of the TCP header fields, here is the detailed explaination.

  • Source port (16 bits) – identifies the sending port
  • Destination port (16 bits) – identifies the receiving port
  • Sequence number (32 bits) – used to ensure correct sequencing of the arriving data
  • Acknowledgment number (32 bits) – next expected TCP octet.
  • Reserved (4 bits) – for future use and should be set to zero
  • Flags (8 bits) (aka Control bits) – contains 8 1-bit flags
  • Window (16 bits) – specifies the number of bytes that the receiver is currently willing to receive
  • Checksum (16 bits) – The 16-bit checksum field is used for error-checking of the header and data
  • Urgent pointer (16 bits) – Indicates the end of urgent data
  • Options (Variable 0-320 bits, divisible by 32) – The length of this field is determined by the data offset field. Options 0 and 1 are a single byte (8 bits) in length. The remaining options indicate the total length of the option (expressed in bytes) in the second byte.

UDP vs. TCP

As seen in the above TCP and UDP segment diagrams, both use port numbers in the source and destination fields, not IP addresses.  These port numbers are used to pass information to upper layers and also to keep track of different simultaneous network conversations.  Port numbers identify the upper layer protocol that is using the transport.

port

IANA controlled some well-known port numbers which programmers agree to use. In the above example, some of the famous well known ports are displayed. port 21 for FTP, port 23 for Telnet, port 25 for SMPT, port 53 for DNS, port 69 for TFTP and port 161 for SNMP.

There are many difference between TCP and UDP.

TCP:

  • Guaranteed delivery
  • Error detection and recovery: Sequence numbers and acknowledgments cover discarding duplicate packets, retransmission of lost packets, and ordered-data transfer. To assure correctness a checksum field is included.
  • Windowing: the receiver specifies in the receive window field the amount of additional received data (in bytes) that it is willing to buffer for the connection. The sending host can send only up to that amount of data before it must wait for an acknowledgment and window update from the receiving host.
  • Connection-oriented: use three-way handshake (SYN, SYN/ACK, ACK)
  • no broadcasting and unicasting service

UDP:

  • best-effort delivery
  • No error detection and recovery
  • No windowing
  • Connectionless
  • support broadcasting and multicasting

ICND1 and ICND2 break down

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ICND1 break down — Network Topologies

Posted on June 30, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |

site moved, check updated contents  Network Topology
 on xyznetwork.blogspot.com

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Network topology refers to the way in which the connections within the network are made.

There are two types of topology: Physical topology and logical topology.

A physical topology defines the way in which the computers, printers, network devices, and other devices are connected. There are three main physical topology: bus, star and ring.

linebus

Pic 1. bus physical topology

A bus topology consists of a main run of cable with a terminator at each end (See pic. 1). All nodes (file server, workstations, and peripherals) are connected to the cable in a line. Ethernet and LocalTalk networks use a bus topology.

star

Pic 2. star physical topology

A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub or concentrator (See pic. 2). This category includes both star and extended-star topologies. Data on a star network passes through the hub or concentrator before continuing to its destination. The hub or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. A physical star topology costs more to implement than the physical bus topology, but physical star topology have an important advantage — if one cable connecting end device to the central device fails, the rest of the network remains operational. This is why star topology is the most common physical topology in Ethernet LANs.

In a ring topology, computers and other network devices are cabled together with the last device connected to the first to form a circle, or ring. This category includes both single-ring and dual-ring topologies. In a single -ring topology, all the devices on the network share a single cable, and the data travels in one direction only. While signe ring is suceptible to a single failure, the dual-ring topology introduces fault tolerance by creating two rings with two cables, which allow data to sent in both diretions. If one ring fails, data can be transmitted on the other ring.

Besides the three physical topologies mentioned above, there are other physical topologies. One example is tree topology.

tree

Pic 3. tree physical topology

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable (See pic. 3). Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs

The logical topology, in contrast,  describes the paths that the signals travel from one point on a network to another without regard to the physical interconnection of the devices.

Logical topologies are bound to the network protocols that direct how the data moves across a network. The Ethernet protocol (layer 1) is a common logical bus topology protocol. LocalTalk (layer 1) is a common logical bus or star topology protocol. IBM’s Token Ring (layer 2) is a common logical ring topology protocol.

Besides bus, star and ring topology, there are other logical topologies such as mesh and tree topologies. The following pictures and tables listed the most common logical topologies.

NetworkTopologies

Mesh Topology Devices are connected with many redundant interconnections between network nodes. In a true mesh topology every node has a connection to every other node in the network. In a partial mesh, at least one device maintains multiple connections to other devices.
Star Topology All devices are connected to a central hub. Nodes communicate across the network by passing data through the hub. When a star network is expanded to include an additional network device that is connected to the main network devices, the topology is referred to as an extended-star topology.
Bus Topology All devices are connected to a central cable, called the bus or backbone. The sending node broadcasts the data to the entire network. The various nodes hear it and look to see if the data is for them. If so, they keep the data. If not they ignore the data.  An example of a logical bus topology is an Ethernet hub.
Ring Topology All devices are connected to one another in the shape of a closed loop. In the Ring Topology, the sending node passes a token or small message around the ring. The computer wish to transmit catches the token, attaches a message to it, and then lets it continue to travel around the network. After one full circle, the sending node will destroy the token. Since only the computer with the token at a particular time may transmit a message so that collision will never occur. The Token Ring and Fiber Distributed Data Interface (FDDI) are examples of a Ring Logical Network
Tree Topology A hybrid topology. Groups of star-configured networks are connected to a linear bus backbone.This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.

A network’s logical topology is not necessarily the same as its physical topology. For example, IBM’s Token Ring is a logical ring topology, it is physically set up in a star topology.

ICND1 and ICND2 break down

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ICND1 break down — Ethernet Connectors and Cable Types

Posted on June 30, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |
















































The cable and connector specifications used to support Ethernet implementations are derived from the EIA/TIA standards body. There are many network connectors:
  • RJ-11 is a physical interface often used for terminating telephone wires. It is probably the most familiar of the registered jacks, being used for single line telephone jacks in most homes across the world.Rj25_connector
  • RJ-45 is slightly larger than RJ-11 phone connectors and jacks.Rj45plug-8p8c
  • Attachment Unit Interface (AUI connectors): is a 15 pin connection that provides a path between a node’s Ethernet interface and the Medium Attachment Unit (MAU), sometimes known as a transceiver.800px-AUI_Connectors
  • Gigabit Interface Converter (GBIC): a hot-swappable I/O device that plugs into a Gigabit Ethernet port. GBICs are used in the LAN for uplinks and are normally used for the backbone. 551px-GBIC
  • fiber-optic GBIC: a transceiver that acts as electrical current/optical signal converter. 281271_gbic
When choosing Ethernet cable types, maximum segment length and speed are the most important factors. The following table compares the most common Ethernet cables.
Cable Type Comparison
Media Type Maximum Segment Length Speed Cost Advantages Disadvantages
UTP 100 m 10 Mbps to 1000 Mbps Least expensive Easy to install; widely available and widely used Susceptible to interference; can cover only a limited distance
STP 100 m 10 Mbps to 100 Mbps More expensive than UTP Reduced crosstalk; more resistant to EMI than Thinnet or UTP Difficult to work with; can cover only a limited distance
Coaxial 500 m (Thicknet)185 m (Thinnet) 10 Mbps to 100 Mbps Relatively inexpensive, but more costly than UTP Less susceptible to EMI interference than other types of copper media Difficult to work with (Thicknet); limited bandwidth; limited application (Thinnet); damage to cable can bring down entire network
Fiber-Optic 10 km and farther (single-mode)2 km and farther (multimode) 100 Mbps to 100 Gbps (single mode)100 Mbps to 9.92 Gbps (multimode) Expensive Cannot be tapped, so security is better; can be used over great distances; is not susceptible to EMI; has a higher data rate than coaxial and twisted-pair cable Difficult to terminate

CCENT have many questions about UTP cable and the the categories of UTP cable, therefore, we will talk more about this topic.

Unshielded Twisted Pair (UTP) is a cable that has four pairs of wires twisted inside it to eliminate electrical interference. EIA/TIA standards body specifies an RJ-45 connector for UTP cable. RJ-45 connector is the most common type of connection media, which have eight connector pins. UTP cable has a small external diameter of 0.43 cm, make it easy to install.

utp

The categories of cabling defined for Ethernet are derived from the EIA/TIA-568 (SP-2840) Commercial Building Telecommunications Wiring standards. Here is the category of UTP cable:

  •  Category 1: Previously used for POTS telephone communications, ISDN and doorbell wiring.
  • Category 2: Previously was frequently used on 4 Mbit/s token ring networks.
  • Category 3: the standard cable for use with Ethernet 10Base-T, capable of transmit data at speeds up to 10 Mbps.
  • Category 4: no longer common or used in new installations, was frequently used on 16 Mbit/s token ring networks.
  • Category 5: standard cable for use with Ethernet 100Base-TX, capable of transmitting data at speeds up to 100 Mbps.
  • Category 5e: standard cable for use with Ethernet 1000Base-T, capable of transmitting data at speeds up to 1000 Mbps.
  • Category 6: Consists of 4 pairs of 24-gauge copper wires, which can transmit data at speeds of up to 1000 Mbps.

ICND1 and ICND2 break down

 

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ICND1 break down — Cable Category (cat 1, cat2, cat3, cat4, cat5, cat6)

Posted on June 29, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |

  • Cat 1: Previously used for POTS telephone communications, ISDN and doorbell wiring.
  • Cat 2: Previously was frequently used on 4 Mbit/s token ring networks.
  • TIA-568 only recognized cables of Category 3 ratings or above.(wikipedia)

    Cat 3 UTP (TechFAQ)

    Category 3 UTP is rated to carry data up to 10Mbit/s.

    Cat 3 UTP was the standard cable for use with Ethernet 10Base-T.

    Cat 4 UTP:

    Category 5 UTP no longer common or used in new installations, was frequently used on 16 Mbit/s token ring networks.

    Cat 5 UTP

    Category 5 UTP is rated to carry Ethernet up to 100Mbit/s and ATM up to 155Mbit/s.

    Cat 5 UTP was the standard cable for use with Ethernet 100Base-TX.

    Cat 5e UTP

    Category 5e UTP is an enhanced version of Cat 5 UTP.

    Cat 5e UTP is rated to carry data up to 1000Mbit/s.

    Cat 5e UTP is the standard cable for use with Ethernet 1000Base-T.

    Cat 5e can also be used to extend the distance of 100Base-TX cable runs up to 350 meters.

    Cat 6 UTP

    Category 6 UTP is very similar to Cat 5 UTP, except that it is designed and manufactured to even stricter standards.

    Category 6 has a minumum of 250 MHz of bandwidth. Allowing 10/100/1000 use with up to 100 meter cable length, along with 10GbE over shorter distances. (Ethernet Cable Identification and Use)

    Here is a video help you to remember the Cable Category

    ICND1 and ICND2 break down

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    ICND1 break down — IP Address Classes

    Posted on June 29, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |

    content moved, check updated IP Address Classes on xyznetwork.blogspot.com

     

     

     

     

     

     

     

     

     

     

     

     

     

    IP addressing – basic, network and host portion, class A,B,C,D,E

    IP addressing – teaches the way computers distinguish different classes of address.

    At the early stage of IP netowk, no classes of addresses existed. As the number of networks grew, the IP addresses were broken into classes to accomodate different sizes of networks.

    It’s better to remember these facts about address classes sooner than later, because we will have to remember them to pass the CCENT test anyway:

    • Class A  (1-126): The Class A address category was designed to support extremely large networks. A class A address uses only the first octet to indicate the network address. The remaining three octets are used for host addresses. The first bit of a Class A address is always 0; therefore, the lowest number that can be represented is 00000000 (decimal 0), and the highest number that can be represented is 01111111 (decimal 127). However, these two network numbers, 0 and 127, are
      reserved and cannot be used as a network address. Any address start with 127 is reserved for loopback.
    • Class B (128-191): The Class B address category was designed to support middle-sized and large-sized networks. A Class B address uses the first two octets to indicate the network address. The remaining two octets are used for host addresses. The first two bits of a Class B address is alsways binary number 10; therefore, the lowest number that can be represented is 10000000 (decimal 128), and the highest number that can be represented is 1011111 (decimal 191).
    • Class C (192-223): The Class C address category is designed to support small-sized networks. A Class C address uses the first three octets to indicate the network address. The remaining one octet is used for host addresses. The first three bits of a Class C address is always binary number 110; therefore the lowest number that can be represented is 11000000 (decimal 192), and the highest number that can be represented is 11011111 (decimal 223).
    • Class D: 224-239 reserved for multicasting, so that a single station can simultaneously transmit a signle stream of datagrams to multiple recipients. The first four bits of Class D address is always binary number 1110.
    • Class E: 240-255 “experimental addresses”, reserved by the Internet Engineering Task Force (IETF) for its won research.

    Note the bits in the first octet identifies the address class (Class A start with 0, Class B start with 10, Class C start with 110, etc.). The router uses the first bits to identify how many bits it must match to interpret the network portion of the address.

    The block at the beginning and end of each class is called network address and broadcast address, respectively. These two special IP addresses are reserved and cannot be assigned to individual devices on a network.

    • Network address: An IP address that has all host bits set to 0. This address identifies the network itself and cannot be assigned to individual devices on a network. For example, 61.0.0.0 is the Network address of the network containing the host 61.4.64.21.
    • Broadcast address: An IP address with all host bits set to 1. Broadcast address, as the name suggests, used to send data to all the devices on a network. For example, 61.255.255.255 is the Broad address of the network containing the host 61.4.64.21. The network broadcast is also known as a directed broadcast and is capable of being routed. The router would forward broadcast packets out all the interfaces with the same network ID. Cisco routers disable broadcast-forwarding by default.

    If an IP device wants to communicate with all devices on all networks, it sends packets to address 255.255.255.255. This is used in RARP and DHCP protocols. An all network broadcast is not capable of being routed, it stays local to LAN segment or VLAN, therefore is also called local broadcast.

    prefix notation — short-hand notation for subnet mask:
    10.0.0.0/8 (/8 means the subnet mask have 8 leading 1s, which is 255.0.0.0)
    172.16.0.0/12 (/12 = 255.240.0.0)

    The default subnet mask:

    • class A — 255.0.0.0 (/8) creating 3 octets for the host field.
    • class B — 255.255.0.0 (/16) creating 2 octets for the host field.
    • class C — 255.255.255.0 (/24) creating 1 octets for the host field.

    To sum up:

    The list of the Class A, B, C, D, E IP address.

    Class Leading bits Start End Default Subnet Mask in dotted decimal
    A (CIDR /8) 0 0.0.0.0 127.255.255.255 255.0.0.0
    B (CIDR /16) 10 128.0.0.0 191.255.255.255 255.255.0.0
    C (CIDR /24) 110 192.0.0.0 223.255.255.255 255.255.255.0
    D 1110 224.0.0.0 239.255.255.255
    E 1111 240.0.0.0 255.255.255.254

    The block at the beginning and end of each class (A, B and C) were designated as reserved for purpose such as future experimentation, internal use in managing the Internet etc. In another words, 0. x.x.x and 127. x.x.x are reserved for class A; 128.0.x.x and 191.255.x.x are reserved for class B; 192.0.0.x and 223.255.255.x are reserved for class C.

    While the 127.0.0.0/8 network is in the Class A area, it is designated for loopback and used for testing purpose and cannot be assigned to a network.

    ICND1 and ICND2 break down

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    ICND1 break down — NAT & PAT

    Posted on June 24, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |

    content moved, check updated NAT and PAT on xyznetwork.blogspot.com

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    Two common problems are tackled here :

    • Internet Security – this has become the most important goal of network administrators.  The first step of any security plan, is to make users anonymous.
    • Internet Addresses – these are limited, and have become a very valuable commodity.

    NAT enhances security by changing the IP address and port of each user, so that the outside world (the Internet) sees them as someone else (much like the Government’s witness protection program).  Their identities are changed, and they become anonymous.  PAT allows groups of users to share one common IP address, which is a Godsend to corporations, small businesses, and the Internet itself, which is running out of available IP addresses. NAT and PAT are very simple, yet extremely powerful concepts.

    Cisco defines these terms as:

    • Inside local address—The IP address assigned to a host on the inside network. This is the address configured as a parameter of the computer OS or received via dynamic address allocation protocols such as DHCP. The address is likely not a legitimate IP address assigned by the Network Information Center (NIC) or service provider.
    • Inside global address—A legitimate IP address assigned by the NIC or service provider that represents one or more inside local IP addresses to the outside world.
    • Outside local address—The IP address of an outside host as it appears to the inside network. Not necessarily a legitimate address, it is allocated from an address space routable on the inside.
    • Outside global address—The IP address assigned to a host on the outside network by the host owner. The address is allocated from a globally routable address or network space.

    NAT (1 to 1 translation) – utilizes Source IP addresses and maps them to outside Internet IP addreses.  As shown in the following NAT example, it takes a network address 10.0.0.1, and “translates” it to another network address 171.69.58.80.  It is a simple lookup table, where each row is created  by a router command with the two addresses.  The user address is behind the router on the LAN interface, and the Internet address is sent out across the serial interface.

    PAT (Many to 1 translation  –  overload) – utilizes Source Port IP addresses and ports to uniquely identify user workstations by their socket. A socket is simply an IP address and a port number.  This allows mapping of up to 65,536 inside “socket” addresses to 1 outside address (hence the term ‘overload’).

    In the above PAT example, suppose local private hosts 10.6.1.2 and 10.6.1.6 both send packets from source port 2000. A PAT device might translate these to a single public IP address 171.69.68.10 but two different source ports, say 2031 and 1506. Response traffic received for port 2031 is routed to 10.6.1.2 while port 1506 traffic is routed to 10.6.1.6.

    The following image shows how 3 users can all communicate on the Internet with just one IP address.  The router shown must be capable of performing NAT:

    nat1

    NAT Overloading Example

    For this example, you have four users (each using non-routable internal network addresses ) behind a router with NAT capability.  The router has one legal IP address, 215.37.32.203, that it advertises to the Internet, but four unique ports.  A remote server may communicate with multiple workstations on this LAN by also using it’s one IP address but four unique ports

    Source
    Computer
    Source
    Computer’s
    IP Address
    Source
    Computer’s
    Port
    NAT Router’s
    IP Address
    NAT Router’s
    Assigned
    Port Number
    A 10.0.0.1 400 215.37.32.203 1
    B 10.0.0.2 50 215.37.32.203 2
    C 10.0.0.3 3750 215.37.32.203 3
    D 10.0.0.4 3750 215.37.32.203 4

    Here’s how the overloading works.  They key is an “address translation table” set up and stored by the router:

    • An internal network (stub domain) has been set up with non-routable IP addresses that were not specifically allocated to that company by IANA.
    • The company sets up a router with NAT enabled. The router has a unique IP address given to the company by IANA.
    • A computer on the stub domain attempts to connect to a computer outside the network, such as a Web server with ip address 175.56.28.03.
    • The router receives the packet from the computer on the stub domain.
    • The router saves the computer’s non-routable IP address and port number to an address translation table. The router replaces the sending computer’s non-routable IP address with the router’s IP address. The router also replaces the sending computer’s source port – it is simplest to use the row number of that entry in the address translation table.  For example, the first entry is for computer A, and that computer’s source port (400) is stored, along with the translated port number ( 1 ).  The translation table now has a mapping of the computer’s non-routable IP address and port numbers along with the router’s IP address. NOTE1:  so now, anyone in the outside world communicating with computer A, will believe that Computer A’s address and port is 215.37.32.203,  port 1  (the router’s address, with port 1).  The router receives the data, translates it to 192.168.32.10,  port 400, and delivers it to Computer A via the Ethernet segment.NOTE2:  the port numbers 1,2,3, and 4 are reserved “well-known” port numbers  (Well-Known ports are those in the range from 1 to 1023).  It is unclear how they can instead be used for the purpose of address translation, but apparently it does not cause problems.
    • When a packet comes back from the destination computer, the router checks the destination port on the packet. It then looks in the address translation table to see which computer on the stub domain the packet belongs to. It changes the destination address and destination port to the one saved in the address translation table and sends it to that computer.
    • The computer receives the packet from the router and the process repeats as long as the computer is communicating with the external system.
    • Since the NAT router now has the computer’s source address and source port saved to the address translation table, it will continue to use that same port number for the duration of the connection. A timer is reset each time the router accesses an entry in the table. If the entry is not accessed again before the timer expires, the entry is removed from the table.

    As you can see, the NAT router stores the IP address and port number of each computer in the address translation table. It then replaces the IP address with its own registered IP address and the port number corresponding to the location of the entry for that packet’s source computer in the table. So any external network sees the NAT Router’s IP address and the port number assigned by the router as the source computer information on each packet.

    You can still have some computers on the stub domain that use dedicated IP addresses. You can create an access list of IP addresses that tells the router which computers on the network require NAT. All other IP addresses will pass through untranslated.

    The number of simultaneous translations that a router will support is determined mainly by the amount of DRAM (Dynamic Random Access Memory) it has. But since a typical entry in the address translation table only takes about 160 bytes, a router with 4 MB of DRAM could theoretically process 26,214 simultaneous translations! Which is more than enough for most applications.

    On the cisco routers, the NAT and PAT translation table can be viewed with the command “show ip nat translations“, the command “clear ip nat translation” clears all dynamic address translation entries from the NAT translation table.

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