Build a Small Network PDF

Welcome to Build a Small Network! Hooray! You have come to the final module in the Introduction to Networks v7.0 course. You have most of the foundational knowledge needed to set up your own network. Where do you go from here? You build a network, of course. And not only do you build one, you verif...

Welcome to Build a Small Network! Hooray! You have come to the final module in the Introduction to Networks v7.0 course. You have most of the foundational knowledge needed to set up your own network. Where do you go from here? You build a network, of course. And not only do you build one, you verify that it is working, and even troubleshoot some common network problems. This module has labs and Packet Tracer activities to help you practice your new skills as a network administrator. Let\'s get going! Small Network Topologies The majority of businesses are small; therefore, it is not surprising that the majority of business networks are also small. A small network design is usually simple. The number and type of devices included are significantly reduced compared to that of a larger network. mall networks typically have a single WAN connection provided by DSL, cable, or an Ethernet connection. Large networks require an IT department to maintain, secure, and troubleshoot network devices and to protect organizational data. Managing a small network requires many of the same skills as those required for managing a larger one. Small networks are managed by a local IT technician or by a contracted professional. Device Selection for a Small Network Like large networks, small networks require planning and design to meet user requirements. Planning ensures that all requirements, cost factors, and deployment options are given due consideration. One of the first design considerations is the type of intermediary devices to use to support the network. **Cost** The cost of a switch or router is determined by its capacity and features. This includes the number and types of ports available and the backplane speed. Other factors that influence the cost are network management capabilities, embedded security technologies, and optional advanced switching technologies. The expense of cable runs required to connect every device on the network must also be considered. Another key element affecting cost considerations is the amount of redundancy to incorporate into the network. **Speed and Types of Ports/Interfaces** Choosing the number and type of ports on a router or switch is a critical decision. Newer computers have built-in 1 Gbps NICs. Some servers may even have 10 Gbps ports. Although it is more expensive, choosing Layer 2 devices that can accommodate increased speeds allows the network to evolve without replacing central devices. **Expandability** Networking devices are available in fixed and modular physical configurations. Fixed configuration devices have a specific number and type of ports or interfaces and cannot be expanded. Modular devices have expansion slots to add new modules as requirements evolve. Switches are available with additional ports for high-speed uplinks. Routers can be used to connect different types of networks. Care must be taken to select the appropriate modules and interfaces for the specific media. **Operating System Features and Services** Network devices must have operating systems that can support the organizations requirements such as the following: - Layer 3 switching - Network Address Translation (NAT) - Dynamic Host Configuration Protocol (DHCP) - Security - Quality of service (QoS) - Voice over IP (VoIP) IP Addressing for a Small Network When implementing a network, create an IP addressing scheme and use it. All hosts and devices within an internetwork must have a unique address. Devices that will factor into the IP addressing scheme include the following: - End user devices - The number and type of connection (i.e., wired, wireless, remote access) - Servers and peripherals devices (e.g., printers and security cameras) - Intermediary devices including switches and access points It is recommended that you plan, document, and maintain an IP addressing scheme based on device type. The use of a planned IP addressing scheme makes it easier to identify a type of device and to troubleshoot problems, as for instance, when troubleshooting network traffic issues with a protocol analyzer. Redundancy in a Small Network Another important part of network design is reliability. Even small businesses often rely heavily on their network for business operation. A failure of the network can be very costly. In order to maintain a high degree of reliability, *redundancy* is required in the network design. Redundancy helps to eliminate single points of failure. There are many ways to accomplish redundancy in a network. Redundancy can be accomplished by installing duplicate equipment, but it can also be accomplished by supplying duplicate network links for critical areas. Small networks typically provide a single exit point toward the internet via one or more default gateways. If the router fails, the entire network loses connectivity to the internet. For this reason, it may be advisable for a small business to pay for a second service provider as backup. Traffic Management The goal for a good network design, even for a small network, is to enhance the productivity of the employees and minimize network downtime. The network administrator should consider the various types of traffic and their treatment in the network design. The routers and switches in a small network should be configured to support real-time traffic, such as voice and video, in an appropriate manner relative to other data traffic. In fact, a good network design will implement quality of service (QoS) to classify traffic carefully according to priority during times of congestion. Common Applications The previous topic discussed the components of a small network, as well as some of the design considerations. These considerations are necessary when you are just setting up a network. After you have set it up, your network still needs certain types of applications and protocols in order to work. The network is only as useful as the applications that are on it. There are two forms of software programs or processes that provide access to the network: network applications and application layer services. **Network Applications** Applications are the software programs used to communicate over the network. Some end-user applications are network-aware, meaning that they implement application layer protocols and are able to communicate directly with the lower layers of the protocol stack. Email clients and web browsers are examples of this type of application. **Application Layer Services** Other programs may need the assistance of application layer services to use network resources like file transfer or network print spooling. Though transparent to an employee, these services are the programs that interface with the network and prepare the data for transfer. Different types of data, whether text, graphics or video, require different network services to ensure that they are properly prepared for processing by the functions occurring at the lower layers of the OSI model. Each application or network service uses protocols, which define the standards and data formats to be used. Without protocols, the data network would not have a common way to format and direct data. In order to understand the function of various network services, it is necessary to become familiar with the underlying protocols that govern their operation. Use the Task Manager to view the current applications, processes, and services running on a Windows PC. Common Protocols Most of a technician's work, in either a small or a large network, will in some way be involved with network protocols. Network protocols support the applications and services used by employees in a small network. Network administrators commonly require access to network devices and servers. The two most common remote access solutions are Telnet and Secure Shell (SSH). SSH service is a secure alternative to Telnet. When connected, administrators can access the SSH server device as though they were logged in locally. SSH is used to establish a secure remote access connection between an SSH client and other SSH-enabled devices: - **Network device** - The network device (e.g., router, switch, access point, etc.) must support SSH to provide remote access SSH server services to clients. - **Server** - The server (e.g., web server, email server, etc.) must support remote access SSH server services to clients. Network administrators must also support common network servers and their required related network protocols. **Web Server** - Web clients and web servers exchange web traffic using the Hypertext Transfer Protocol (HTTP). - Hypertext Transfer Protocol Secure (HTTPS) is used for secure web communication. **Email Server** - Email servers and clients use Simple Mail Transfer Protocol (SMTP) to send emails. - Email clients use Post Office Protocol (POP3) or Internet Message Access Protocol (IMAP) to retrieve email. - Recipients are specified using the  format. **FTP Server** - File Transfer Protocol (FTP) service allows files to be downloaded and uploaded between a client and FTP server. - FTP Secure (FTPS) and Secure FTP (SFTP) are used to secure FTP file exchange. **DHCP Server** Dynamic Host Configuration Protocol (DHCP) is used by clients to acquire an IP configuration (i.e., IP address, subnet mask, default gateway and more) from a DHCP server. **DNS Server** - Domain Name System (DNS) resolves a domain name to an IP address (e.g., [cisco.com](http://cisco.com/) = 72.163.4.185) - DNS provides the IP address of a web site (i.e., domain name) to a requesting host. **Note:** A server could provide multiple network services. For instance, a server could be an email, FTP, and SSH server. These network protocols comprise the fundamental toolset of a network professional. Each of these network protocols define: - Processes on either end of a communication session - Types of messages - Syntax of the messages - Meaning of informational fields - How messages are sent and the expected response - Interaction with the next lower layer Many companies have established a policy of using secure versions (e.g., SSH, SFTP, and HTTPS) of these protocols whenever possible. Voice and Video Applications Businesses today are increasingly using IP telephony and streaming media to communicate with customers and business partners. Many organizations are enabling their employees to work remotely. As the figure shows, many of their users still require access to corporate software and files, as well as support for voice and video applications. The network administrator must ensure the proper equipment is installed in the network and that the network devices are configured to ensure priority delivery. **Infrastructure** - The network infrastructure must support the real-time applications. - Existing devices and cabling must be tested and validated. - Newer networking products may be required. **VoIP** - VoIP devices convert analog telephone signals into digital IP packets. - Typically, VOIP is less expensive than an IP telephony solution, but the quality of communications does not meet the same standards. - Small network voice and video over IP can be solved using Skype and non-enterprise versions of Cisco WebEx. **IP Telephony** - An IP phone performs voice-to-IP conversion with the use of a dedicated server for call control and signaling. - Many vendors provide small business IP telephony solutions such as the Cisco Business Edition 4000 Series products. **Real-Time Applications** - The network must support quality of service (QoS) mechanisms to minimize latency issues for real-time streaming applications. - Real-Time Transport Protocol (RTP) and Real-Time Transport Control Protocol (RTCP) are two protocols that support this requirement. Small Network Growth If your network is for a small business, presumably, you want that business to grow, and your network to grow along with it. This is called scaling a network, and there are some best practices for doing this. Growth is a natural process for many small businesses, and their networks must grow accordingly. Ideally, the network administrator has enough lead-time to make intelligent decisions about growing the network in alignment with the growth of the company. To scale a network, several elements are required: - **Network documentation** - Physical and logical topology - **Device inventory** - List of devices that use or comprise the network - **Budget** - Itemized IT budget, including fiscal year equipment purchasing budget - **Traffic analysis** - Protocols, applications, and services and their respective traffic requirements should be documented These elements are used to inform the decision-making that accompanies the scaling of a small network. Protocol Analysis As the network grows, it becomes important to determine how to manage network traffic. It is important to understand the type of traffic that is crossing the network as well as the current traffic flow. There are several network management tools that can be used for this purpose. However, a simple protocol analyzer such as Wireshark can also be used. For instance, running Wireshark on several key hosts can reveal the types of network traffic flowing through the network. To determine traffic flow patterns, it is important to do the following: - Capture traffic during peak utilization times to get a good representation of the different traffic types. - Perform the capture on different network segments and devices as some traffic will be local to a particular segment. Information gathered by the protocol analyzer is evaluated based on the source and destination of the traffic, as well as the type of traffic being sent. This analysis can be used to make decisions on how to manage the traffic more efficiently. This can be done by reducing unnecessary traffic flows or changing flow patterns altogether by moving a server, for example. Sometimes, simply relocating a server or service to another network segment improves network performance and accommodates the growing traffic needs. At other times, optimizing the network performance requires major network redesign and intervention. Employee Network Utilization In addition to understanding changing traffic trends, a network administrator must be aware of how network use is changing. Many operating systems provide built-in tools to display such information. For example, a Windows host provides tools such as the Task Manager, Event Viewer, and Data Usage tools. These tools can be used to capture a "snapshot" of information such as the following: - OS and OS Version - CPU utilization - RAM utilization - Drive utilization - Non-Network applications - Network applications Documenting snapshots for employees in a small network over a period of time is very useful to identify evolving protocol requirements and associated traffic flows. A shift in resource utilization may require the network administrator to adjust network resource allocations accordingly. The Windows 10 Data Usage tool is especially useful to determine which applications are using network services on a host. The Data Usage tool is accessed using **Settings \> Network & Internet \> Data usage \> network interface** (from the last 30 days). Verify Connectivity with Ping Whether your network is small and new, or you are scaling an existing network, you will always want to be able to verify that your components are properly connected to each other and to the internet. This topic discusses some utilities that you can use to ensure that your network is connected. The **ping** command is the most effective way to quickly test Layer 3 connectivity between a source and destination IP address. The command also displays various round-trip time statistics. Specifically, the **ping** command uses the Internet Control Message Protocol (ICMP) echo request (ICMP Type 8) and echo reply (ICMP Type 0) messages. The **ping** command is available in most operating systems including Windows, Linux, macOS, and Cisco IOS. On a Windows 10 host, the **ping** command sends four consecutive ICMP echo request messages and expects four consecutive ICMP echo replies from the destination. Extended Ping A standard **ping** uses the IP address of the interface closest to the destination network as the source of the **ping**. The source IP address of the **ping 10.1.1.10** command on R1 would be that of the G0/0/0 interface (i.e., 209.165.200.225). The Cisco IOS offers an \"extended\" mode of the **ping** command. This mode enables the user to create special type of pings by adjusting parameters related to the command operation. Extended ping is entered in privileged EXEC mode by typing **ping** without a destination IP address. You will then be given several prompts to customize the extended **ping**. **Note:** Pressing **Enter** accepts the indicated default values. For example, assume you wanted to test connectivity from the R1 LAN (i.e., 192.168.10.0/24) to the 10.1.1.0 LAN. This could be verified from the PC A. However, an extended **ping** could be configured on R1 to specify a different source address. Verify Connectivity with Traceroute The **ping** command is useful to quickly determine if there is a Layer 3 connectivity problem. However, it does not identify where the problem is located along the path. Traceroute can help locate Layer 3 problem areas in a network. A trace returns a list of hops as a packet is routed through a network. It could be used to identify the point along the path where the problem can be found. The syntax of the trace command varies between operating systems, **Note:** Use **Ctrl-C** to interrupt a **tracert** in Windows. Extended Traceroute Like the extended **ping** command, there is also an extended **traceroute** command. It allows the administrator to adjust parameters related to the command operation. This is helpful in locating the problem when troubleshooting routing loops, determining the exact next-hop router, or determining where packets are getting dropped or denied by a router or firewall. The Windows **tracert** command allows the input of several parameters through options in the command line. However, it is not guided like the extended traceroute IOS command.  The Cisco IOS extended **traceroute** option enables the user to create a special type of trace by adjusting parameters related to the command operation. Extended traceroute is entered in privileged EXEC mode by typing **traceroute** without a destination IP address. IOS will guide you through the command options by presenting a number of prompts related to the setting of all the different parameters. **Note:** Pressing **Enter** accepts the indicated default values. Network Baseline One of the most effective tools for monitoring and troubleshooting network performance is to establish a network baseline. Creating an effective network performance baseline is accomplished over a period of time. Measuring performance at varying times and loads will assist in creating a better picture of overall network performance. The output derived from network commands contributes data to the network baseline. One method for starting a baseline is to copy and paste the results from an executed **ping**, **trace**, or other relevant commands into a text file. These text files can be time stamped with the date and saved into an archive for later retrieval and comparison. Among items to consider are error messages and the response times from host to host. If there is a considerable increase in response times, there may be a latency issue to address. Notice this time that the **ping** round-trip times are much longer indicating a potential problem. Corporate networks should have extensive baselines; more extensive than we can describe in this course. Professional-grade software tools are available for storing and maintaining baseline information. In this course, we cover a few basic techniques and discuss the purpose of baselines. Cisco's best practices for baseline processes can be found by searching the internet for "Baseline Process Best Practices". IP Configuration on a Windows Host If you have used any of the tools in the previous topic to verify connectivity and found that some part of your network is not working as it should, now is the time to use some commands to troubleshoot your devices. Host and IOS commands can help you determine if the problem is with the IP addressing of your devices, which is a common network problem. Checking the IP addressing on host devices is a common practice in networking for verifying and troubleshooting end-to-end connectivity. In Windows 10, you can access the IP address details from the **Network and Sharing Center**, as shown in the figure, to quickly view the four important settings: address, mask, router, and DNS. However, network administrators typically view the IP addressing information on a Windows host by issuing the **ipconfig** command at the command line of a Windows computer. Use the **ipconfig /all** command to view the MAC address, as well as a number of details regarding the Layer 3 addressing of the device. If a host is configured as a DHCP client, the IP address configuration can be renewed using the **ipconfig /release** and **ipconfig /renew** commands. The DNS Client service on Windows PCs also optimizes the performance of DNS name resolution by storing previously resolved names in memory. The **ipconfig /displaydns** command displays all of the cached DNS entries on a Windows computer system. IP Configuration on a Linux Host Verifying IP settings using the GUI on a Linux machine will differ depending on the Linux distribution (distro) and desktop interface. On the command line, network administrators use the **ifconfig** command to display the status of the currently active interfaces and their IP configuration. The Linux **ip address** command is used to display addresses and their properties. It can also be used to add or delete IP addresses. **Note:** The output displayed may vary depending on the Linux distribution. The arp Command The **arp** command is executed from the Windows, Linux, or Mac command prompt. The command lists all devices currently in the ARP cache of the host, which includes the IPv4 address, physical address, and the type of addressing (static/dynamic), for each device. The **arp -a** command displays the known IP address and MAC address binding. Notice how IP address 10.0.0.5 is not included in the list. This is because the ARP cache only displays information from devices that have been recently accessed. To ensure that the ARP cache is populated, **ping** a device so that it will have an entry in the ARP table. For instance, if PC-A pinged 10.0.0.5, then the ARP cache would contain an entry for that IP address. The cache can be cleared by using the **netsh interface ip delete arpcache** command in the event the network administrator wants to repopulate the cache with updated information. **Note:** You may need administrator access on the host to be able to use the **netsh interface ip delete arpcache** command. Common show Commands Revisited In the same way that commands and utilities are used to verify a host configuration, commands can be used to verify the interfaces of intermediary devices. The Cisco IOS provides commands to verify the operation of router and switch interfaces. The Cisco IOS CLI **show** commands display relevant information about the configuration and operation of the device. Network technicians use **show** commands extensively for viewing configuration files, checking the status of device interfaces and processes, and verifying the device operational status. The status of nearly every process or function of the router can be displayed using a **show** command. Common IOS show commands used in Cisco networking include: 1. **show running-config**: Displays the current configuration running on the device. 2. **show startup-config**: Shows the configuration stored in NVRAM that will be used on the next reboot. 3. **show version**: Provides information about the IOS version, system uptime, and hardware details. 4. **show interfaces**: Displays detailed information about all network interfaces. 5. **show ip interface brief**: Gives a summary of the status and IP addresses of all interfaces. 6. **show arp**: Shows the ARP table, mapping IP addresses to MAC addresses. 7. **show mac address-table**: Displays the MAC address table used for forwarding decisions. 8. **show vlan**: Lists VLAN information and status. 9. **show logging**: Displays the logging configuration and system messages. 10. **show ip route**: Shows the routing table. These commands are essential for network troubleshooting and management The show cdp neighbors Command There are several other IOS commands that are useful. The Cisco Discovery Protocol (CDP) is a Cisco proprietary protocol that runs at the data link layer. Because CDP operates at the data link layer, two or more Cisco network devices, such as routers that support different network layer protocols, can learn about each other even if Layer 3 connectivity has not been established. When a Cisco device boots, CDP starts by default. CDP automatically discovers neighboring Cisco devices running CDP, regardless of which Layer 3 protocol or suites are running. CDP exchanges hardware and software device information with its directly connected CDP neighbors. CDP provides the following information about each CDP neighbor device: - **Device identifiers** - The configured host name of a switch, router, or other device - **Address list** - Up to one network layer address for each protocol supported - **Port identifier** - The name of the local and remote port in the form of an ASCII character string, such as FastEthernet 0/0 - **Capabilities list** - For example, whether a specific device is a Layer 2 switch or a Layer 3 switch - **Platform** - The hardware platform of the device\--for example, a Cisco 1841 series router. The **show cdp neighbors detail** command reveals the IP address of a neighboring device, as shown in the output. CDP will reveal the IP address of the neighbor regardless of whether or not you can ping that neighbor. This command is very helpful when two Cisco routers cannot route across their shared data link. The **show cdp neighbors detail** command will help determine if one of the CDP neighbors has an IP configuration error. As helpful as CDP is, it can also be a security risk because it can provide useful network infrastructure information to threat actors. For example, by default many IOS versions send CDP advertisements out all enabled ports. However, best practices suggest that CDP should be enabled only on interfaces that are connecting to other infrastructure Cisco devices. CDP advertisements should be disabled on user-facing ports. Because some IOS versions send out CDP advertisements by default, it is important to know how to disable CDP. To disable CDP globally, use the global configuration command **no cdp run**. To disable CDP on an interface, use the interface command **no cdp enable**. The show ip interface brief Command One of the most frequently used commands is the **show ip interface brief** command. This command provides a more abbreviated output than the **show ip interface** command. It provides a summary of the key information for all the network interfaces on a router. For example, the **show ip interface brief** output displays all interfaces on the router, the IP address assigned to each interface, if any, and the operational status of the interface. Basic Troubleshooting Approaches In the previous two topics, you learned about some utilities and commands that you can use to help identify problem areas in your network. This is an important part of troubleshooting. There are many ways to troubleshoot a network problem. This topic details a structured troubleshooting process that can help you to become a better network administrator. It also provides a few more commands to help you resolve problems. Network problems can be simple or complex, and can result from a combination of hardware, software, and connectivity issues. Technicians must be able to analyze the problem and determine the cause of the error before they can resolve the network issue. This process is called troubleshooting. A common and efficient troubleshooting methodology is based on the scientific method. To assess the problem, determine how many devices on the network are experiencing the problem. If there is a problem with one device on the network, start the troubleshooting process at that device. If there is a problem with all devices on the network, start the troubleshooting process at the device where all other devices are connected. You should develop a logical and consistent method for diagnosing network problems by eliminating one problem at a time. Resolve or Escalate? In some situations, it may not be possible to resolve the problem immediately. A problem should be escalated when it requires a manager decision, some specific expertise, or network access level unavailable to the troubleshooting technician. For example, after troubleshooting, the technician concludes a router module should be replaced. This problem should be escalated for manager approval. The manager may have to escalate the problem again as it may require the approval of the financial department before a new module can be purchased. A company policy should clearly state when and how a technician should escalate a problem. The debug Command OS processes, protocols, mechanisms and events generate messages to communicate their status. These messages can provide valuable information when troubleshooting or verifying system operations. The IOS **debug** command allows the administrator to display these messages in real-time for analysis. It is a very important tool for monitoring events on a Cisco IOS device. All **debug** commands are entered in privileged EXEC mode. The Cisco IOS allows for narrowing the output of **debug** to include only the relevant feature or subfeature. This is important because debugging output is assigned high priority in the CPU process and it can render the system unusable. For this reason, use **debug** commands only to troubleshoot specific problems. For example, to monitor the status of ICMP messages in a Cisco router, use **debug ip icmp**. Be cautious using some **debug** command. Commands such as **debug all** and **debug ip packet** generate a substantial amount of output and can use a large portion of system resources. The router could get so busy displaying **debug** messages that it would not have enough processing power to perform its network functions, or even listen to commands to turn off debugging. For this reason, using these command options is not recommended and should be avoided. The terminal monitor Command Connections to grant access to the IOS command line interface can be established in the following two ways: - **Locally** - Local connections (i.e., console connection) require physical access to the router or switch console port using a rollover cable. - **Remotely** - Remote connections require the use of Telnet or SSH to establish a connection to an IP configured device. Certain IOS messages are automatically displayed on a console connection but not on a remote connection. For instance, **debug** output is displayed by default on console connections. However, **debug** output is not automatically displayed on remote connections. This is because **debug** messages are log messages which are prevented from being displayed on vty lines. In the following output for instance, the user established a remote connection using Telnet from R2 to R1. The user then issued the **debug ip icmp** command. However, the command failed to display **debug** output. Duplex Operation and Mismatch Issues Many common network problems can be identified and resolved with little effort. Now that you have the tools and the process for troubleshooting a network, this topic reviews some common networking issues that you are likely to find as a network administrator. In data communications, *duplex* refers to the direction of data transmission between two devices. There are two duplex communication modes: - **Half-duplex** - Communication is restricted to the exchange of data in one direction at a time. - **Full-duplex** - Communications is permitted to be sent and received simultaneously. Interconnecting Ethernet interfaces must operate in the same duplex mode for best communication performance and to avoid inefficiency and latency on the link. The Ethernet autonegotiation feature facilitates configuration, minimizes problems and maximizes link performance between two interconnecting Ethernet links. The connected devices first announce their supported capabilities and then choose the highest performance mode supported by both ends. For example, the switch and router in the figure have successfully autonegotiated full-duplex mode. If one of the two connected devices is operating in full-duplex and the other is operating in half-duplex, a duplex mismatch occurs. While data communication will occur through a link with a duplex mismatch, link performance will be very poor. Duplex mismatches are typically caused by a misconfigured interface or in rare instances by a failed autonegotiation. Duplex mismatches may be difficult to troubleshoot as the communication between devices still occurs. IP Addressing Issues on IOS Devices IP address-related problems will likely keep remote network devices from communicating. Because IP addresses are hierarchical, any IP address assigned to a network device must conform to that range of addresses in that network. Wrongly assigned IP addresses create a variety of issues, including IP address conflicts and routing problems. Two common causes of incorrect IPv4 assignment are manual assignment mistakes or DHCP-related issues. Network administrators often have to manually assign IP addresses to devices such as servers and routers. If a mistake is made during the assignment, then communications issues with the device are very likely to occur. On an IOS device, use the **show ip interface** or **show ip interface brief** commands to verify what IPv4 addresses are assigned to the network interfaces. For example, issuing the **show ip interface** **brief** command as shown would validate the interface status on R1. IP Addressing Issues on End Devices In Windows-based machines, when the device cannot contact a DHCP server, Windows will automatically assign an address belonging to the 169.254.0.0/16 range. This feature is called Automatic Private IP Addressing (APIPA) and is designed to facilitate communication within the local network. Think of it as Windows saying, "I will use this address from the 169.254.0.0/16 range because I could not get any other address". Often, a computer with an APIPA address will not be able to communicate with other devices in the network because those devices will most likely not belong to the 169.254.0.0/16 network. This situation indicates an automatic IPv4 address assignment problem that should be fixed. **Note:** Other operating systems, such Linux and OS X, will not assign an IPv4 address to the network interface if communication with a DHCP server fails. Most end devices are configured to rely on a DHCP server for automatic IPv4 address assignment. If the device is unable to communicate with the DHCP server, then the server cannot assign an IPv4 address for the specific network and the device will not be able to communicate. To verify the IP addresses assigned to a Windows-based computer, use the **ipconfig** command. Default Gateway Issues The default gateway for an end device is the closest networking device that can forward traffic to other networks. If a device has an incorrect or nonexistent default gateway address, it will not be able to communicate with devices in remote networks. Because the default gateway is the path to remote networks, its address must belong to the same network as the end device. The address of the default gateway can be manually set or obtained from a DHCP server. Similar to IPv4 addressing issues, default gateway problems can be related to misconfiguration (in the case of manual assignment) or DHCP problems (if automatic assignment is in use). To solve misconfigured default gateway issues, ensure that the device has the correct default gateway configured. If the default address was manually set but is incorrect, simply replace it with the proper address. If the default gateway address was automatically set, ensure the device can communicate with the DHCP server. It is also important to verify that the proper IPv4 address and subnet mask were configured on the interface of the router and that the interface is active. To verify the default gateway on Windows-based computers, use the **ipconfig**. Troubleshooting DNS Issues Domain Name System (DNS) defines an automated service that matches names, such as [www.cisco.com](http://www.cisco.com/), with the IP address. Although DNS resolution is not crucial to device communication, it is very important to the end user. It is common for users to mistakenly relate the operation of an internet link to the availability of the DNS. User complaints such as "the network is down" or "the internet is down" are often caused by an unreachable DNS server. While packet routing and all other network services are still operational, DNS failures often lead the user to the wrong conclusion. If a user types in a domain name such as [www.cisco.com](http://www.cisco.com/) in a web browser and the DNS server is unreachable, the name will not be translated into an IP address and the website will not display. DNS server addresses can be manually or automatically assigned. Network administrators are often responsible for manually assigning DNS server addresses on servers and other devices, while DHCP is used to automatically assign DNS server addresses to clients. Although it is common for companies and organizations to manage their own DNS servers, any reachable DNS server can be used to resolve names. Small office and home office (SOHO) users often rely on the DNS server maintained by their ISP for name resolution. ISP-maintained DNS servers are assigned to SOHO customers via DHCP. Additionally, Google maintains a public DNS server that can be used by anyone and it is very useful for testing. The IPv4 address of Google's public DNS server is 8.8.8.8 and 2001:4860:4860::8888 for its IPv6 DNS address. Cisco offers OpenDNS which provides secure DNS service by filtering phishing and some malware sites. You can change your DNS address to 208.67.222.222 and 208.67.220.220 in the Preferred DNS server and Alternate DNS server fields. Advanced features such as web content filtering and security are available to families and businesses. Use the **ipconfig /all** as shown to verify which DNS server is in use by the Windows computer. The **nslookup** command is another useful DNS troubleshooting tool for PCs. With **nslookup** a user can manually place DNS queries and analyze the DNS response. The **nslookup** command shows the output for a query for [www.cisco.com](http://www.cisco.com/). Notice you can also simply enter an IP address and **nslookup** will resolve the name. **Note:** It is not always possible to type an IP address in **nslookup** and receive the domain name. One of the most common reasons for this is that most websites run on servers that support multiple sites. What did I learn in this module? **Devices in a Small Network** Small networks typically have a single WAN connection provided by DSL, cable, or an Ethernet connection. Small networks are managed by a local IT technician or by a contracted professional. Factors to consider when selecting network devices for a small network are cost, speed and types of ports/interfaces, expandability, and OS features and services. When implementing a network, create an IP addressing scheme and use it on end devices, servers and peripherals, and intermediary devices. Redundancy can be accomplished by installing duplicate equipment, but it can also be accomplished by supplying duplicate network links for critical areas. The routers and switches in a small network should be configured to support real-time traffic, such as voice and video, in an appropriate manner relative to other data traffic. In fact, a good network design will implement quality of service (QoS) to classify traffic carefully according to priority. **Small Network Applications and Protocols** There are two forms of software programs or processes that provide access to the network: network applications and application layer services. Some end-user applications implement application layer protocols and are able to communicate directly with the lower layers of the protocol stack. Email clients and web browsers are examples of this type of application. Other programs may need the assistance of application layer services to use network resources like file transfer or network print spooling. These are the programs that interface with the network and prepare the data for transfer. The two most common remote access solutions are Telnet and Secure Shell (SSH). SSH service is a secure alternative to Telnet. Network administrators must also support common network servers and their required related network protocols such as web server, email server, FTP server, DHCP server, and DNS server. Businesses today are increasingly using IP telephony and streaming media to communicate with customers and business partners. These are real-time applications. The network infrastructure must support VoIP, IP telephony, and other real-time applications. **Scale to Larger Networks** To scale a network, several elements are required: network documentation, device inventory, budget, and traffic analysis. Know the type of traffic that is crossing the network as well as the current traffic flow. Capture traffic during peak utilization times to get a good representation of the different traffic types and perform the capture on different network segments and devices as some traffic will be local to a particular segment. Network administrators must know how network use is changing. Usage details of employee computers can be captured in a 'snapshot' with such tools as the Windows Task Manager, Event Viewer, and Data Usage. **Verify Connectivity** The **ping** command is the most effective way to quickly test Layer 3 connectivity between a source and destination IP address. The command also displays various round-trip time statistics. The Cisco IOS offers an \"extended\" mode of the ping command which lets the user create special types of pings by adjusting parameters related to the command operation. Extended ping is entered in privileged EXEC mode by typing ping without a destination IP address. Traceroute can help locate Layer 3 problem areas in a network. A trace returns a list of hops as a packet is routed through a network. It is used to identify the point along the path where the problem can be found. In Windows, the command is **tracert**. In Cisco IOS the command is **traceroute**. There is also an extended **traceroute** command. It allows the administrator to adjust parameters related to the command operation. The output derived from network commands contributes data to the network baseline. One method for starting a baseline is to copy and paste the results from an executed ping, trace, or other relevant commands into a text file. These text files can be time stamped with the date and saved into an archive for later retrieval and comparison. **Host and IOS Commands** Network administrators view the IP addressing information (address, mask, router, and DNS) on a Windows host by issuing the **ipconfig** command. Other necessary commands are **ipconfig /all**, **ipconfig /release** and **ipconfig /renew**, and **ipconfig /displaydns**. Verifying IP settings by using the GUI on a Linux machine will differ depending on the Linux distribution (distro) and desktop interface. Necessary commands are **ifconfig**, and **ip address**. In the GUI of a Mac host, open Network Preferences \> Advanced to get the IP addressing information. Other IP addressing commands for Mac are **ifconfig**, and **networksetup -listallnetworkservices** and **networksetup -getinfo** \. The **arp** command is executed from the Windows, Linux, or Mac command prompt. The command lists all devices currently in the ARP cache of the host, which includes the IPv4 address, physical address, and the type of addressing (static/dynamic), for each device. The **arp -a** command displays the known IP address and MAC address binding. Common **show** commands are **show running-config**, **show interfaces**, **show ip address**, **show arp**, **show ip route**, **show protocols**, and **show version**. The **show cdp neighbor** command provides the following information about each CDP neighbor device: identifiers, address list, port identifier, capabilities list, and platform. The **show cdp neighbors detail** command will help determine if one of the CDP neighbors has an IP configuration error. The **show ip interface brief** command output displays all interfaces on the router, the IP address assigned to each interface, if any, and the operational status of the interface. **Troubleshooting Methodologies** Step 1. Identify the problem. Step 2. Establish a theory of probably causes. Step 3. Test the theory to determine the cause. Step 4. Establish a plan of action and implement the solution. Step 5. Verify the solution and implement preventive measures. Step 6. Document findings, actions, and outcomes. A problem should be escalated when it requires a the decision of a manager, some specific expertise, or network access level unavailable to the troubleshooting technician. OS processes, protocols, mechanisms and events generate messages to communicate their status. The IOS **debug** command allows the administrator to display these messages in real-time for analysis. To display log messages on a terminal (virtual console), use the **terminal monitor** privileged EXEC command. **Troubleshooting Scenarios** There are two duplex communication modes: half-duplex and full-duplex. If one of the two connected devices is operating in full-duplex and the other is operating in half-duplex, a duplex mismatch occurs. While data communication will occur through a link with a duplex mismatch, link performance will be very poor. Wrongly assigned IP addresses create a variety of issues, including IP address conflicts and routing problems. Two common causes of incorrect IPv4 assignment are manual assignment mistakes or DHCP-related issues. Most end devices are configured to rely on a DHCP server for automatic IPv4 address assignment. If the device is unable to communicate with the DHCP server, then the server cannot assign an IPv4 address for the specific network and the device will not be able to communicate. The default gateway for an end device is the closest networking device that can forward traffic to other networks. If a device has an incorrect or nonexistent default gateway address, it will not be able to communicate with devices in remote networks. Because the default gateway is the path to remote networks, its address must belong to the same network as the end device. DNS failures often lead the user to conclude that the network is down. If a user types in a domain name such as [www.cisco.com](http://www.cisco.com/) in a web browser and the DNS server is unreachable, the name will not be translated into an IP address and the website will not display.