Computer networks Part III
Jul 16, 2009 8:17 AM, By Russell Brown
This tutorial continues with the discussion of what components make a computer network work.
DNS
Domain Name Servers (DNS) are computers that reside on wide area networks that translate human-readable addresses into IP addresses. For example, the URL is the human-readable address such as www.broadcastengineering.com. These names are much easier to remember than a string of numbers for an IP address, but an IP address is required to locate the sites Web servers on the Internet. When you type in a name like www.broadcastengineering.com, your Internet browser knows to send a request for the IP of BroadcastEngineering.com. TCP/IP uses the IP address in the DNS location to send the request. The DNS computer looks up the name BroadcastEngineering.com and returns the IP address associated with it. The computer now has the correct IP address and can send messages to BroadcastEngineering.com asking for its main Web page. This happens each time you type a new URL into your browser.
ARP
The request the computer in the aforementioned example sent out was called an ARP request, which is the method devices use to find out another device’s MAC address when only its IP is known. As a device receives an ARP or an answer to its own ARP request, it builds an ARP cache or memory of the IP addresses and associated MAC addresses. Network switches monitor all traffic and build their own CAM, so they know which MAC-addressed device is attached to which of its ports.
Because network devices can be swapped out and to keep the cache from being outdated, APR caches are cleared on a regular basis and new ARP requests must be sent out to obtain the MAC address again. Remember, on an Ethernet network each IP packet sent has to have the IP and MAC address of its destination, and one data transfer may require anywhere from one to thousands of packets to send all the data. Once the first packet is addressed correctly, all the subsequent ones are addressed similarly using the ARP cache.
Figure 2: Accessing the ARP table on a PC.
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Try this
Figure 3: Checking the IP configuration on a PC.
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Here are several tests you can try on your computer to actually see some of the things covered in the last few newsletters.
Figure 4: Using terminal on a Mac computer to access the ARP table.
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Checking the ARP cache and the IP configuration of your computers can be a valuable troubleshooting tool. Checking both is quite easy on a PC or Macintosh computer.
- ARP on the PC
Under the Start menu select Run. A new window will open; type in “CMD” then hit enter. Type “ARP –A” with a space between “ARP” and “–A.” You will see a list of all known IP addresses with their associated MAC address. (See Figure 2.) (Note: This only works properly on a wired network.)
- IP configuration on the PC
Under the Start menu select Run. A new window will open; type in “CMD” then hit enter. Type “ipconfig.” Now you will see the IP configuration of this computer with IP address, subnet mask, default gateway, primary DNS and secondary DNS. (See Figure 3.)
- ARP on the Macintosh
Figure 5: How to ping on a PC.
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Start the Terminal program from the Utilities folder in Applications. Type “ARP –A” and you will see a list of all known IP addresses with their associated MAC addresses. (See Figure 4.)
- IP configuration on the Macintosh
Figure 6: Pinging on a Mac computer.
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Start the Network Utility program from the Utilities folder in Applications. From the bar at the top of the window, choose Info and you will see the present configuration of all the network interfaces on the computer including IP and MAC address.
- Pinging
Pinging is a simple test you can perform to find out if a particular IP address is in use on a network. When you ping an IP address, you are asking the device that uses it to respond; this lets you know it is on the network and working. Some devices can be programmed to not respond to a ping, but this is uncommon.
- Ping on a PC
Under the Start menu select Run. A new window will open; type in “CMD” and hit enter. Type “ping ###.###.##.###.” Replace the # with the IP address you are looking for. Try an IP address from the ARP command above. (See Figure 5.)
- Ping on a Macintosh
Start the Network Utility program from the Utilities folder in Applications. From the bar at the top of the window, choose Ping and type in the IP address and hit start. (See Figure 6.)
Next Time
The next “Transition to Digital” tutorial will explore troubleshooting a real-world computer network problem.
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