Network Routing - Simple Networking Routing and Routers

This section will explain routing in simple terms with some simple standard rules. There may be exceptions to these rules, but for introductory purposes we will keep the first example simple. Please be aware, that the examples in this section are working examples, but more complexity may be added when a larger network is considered, and multiple data routes become available. Each network interface card (NIC) has a specific address which is an IP address or number. When data is sent between two computers, the data must be sent in a package that has the address of the intended receiver (IP) on it. It is like an envelope (ethernet) with the sender's and recipient's address on it. There is somewhat of a difference, however. When the computer intends to send a packet, it first checks its routing table to see if the intended data must be sent through a gateway. Many computers only have a simple routing table, which is built from the network mask and the gateway information entered, when you set your computer up to do networking. The computer, when set up for networking, must be assigned an IP address, netmask, and default gateway. This may be done manually or done automatically using Dynamic Host Configuration Protocol (DHCP) to assign this information to the computer when it boots. DCHP is described in another section. If the computer determines that the packet must be sent to a gateway, it puts it in a special packet (ethernet) for that gateway, with the actual recipient's address wrapped inside. In the above paragraph, data packets are equated to a letter with an envelope. For this type of thinking, the envelope would be similar to the ethernet, SLIP, or PPP packet which encapsulates the IP packet. The IP packet and its encapsulated data would similar to a letter. Here's generally what happens when a package is sent: The sending computer checks the IP part of the package to see the sender's IP address, and based on the address and instructions in its routing table will do one of the following: Send the packet to the ethernet address of the intended recipient. The following will happen: The ethernet card on the receiving computer will accept the packet. The other network levels (IP, TCP) will open the packet and use it according to filtering and other programming instructions. Send the packet to the ethernet address of a router, depending on the instructions in the routing table. The ethernet card on the router will accept the packet. The IP level of the router will look at the packet's IP address and determine according to its routing table where to send the packet next. It should send it to another router or to the actual recipient. The router will encapsulate the IP packet in another ethernet packet with the ethernet address of the next router or the intended recipient. Router hops will continue until the packet is sent on a network where the intended recipient is physically located unless the packet expires. The ethernet card on the receiving computer will accept the packet. The other network levels (IP, TCP) will open the packet and use it according to filtering and other programming instructions.

Lets say you enter an IP address of 10.1.20.45 and a netmask of 255.255.0.0. This means you are on the network 10.1.0.0 (I show it as 10.1.x.x, the X's mean don't care conditions). The machine's IP address and netmask, together define the network, that it's NIC is on. Therefore any machine that fits in the address range provided under 10.1.x.x can be accessed directly from your NIC, and any that are not in this number range, such as 10.3.34.67 cannot be accessed directly and must be sent to a gateway machine since it is on another network. Typically most machines will use their netmask to make this determination which means if the address does not match their known network, the package will be sent to that machine's default gateway in a special package meant for a router. It works similar to a post office. When you send a letter in your town, you put it in the local slot. It can be delivered to someone else in your town (network), but if you are sending to another town (network), you put the letter in the out of town slot (default gateway), then the mail personnel put it in a special container or box and send it to a main town (gateway), which then decides where to send it based on its address. Although this simple network and default gateway may be common, specific computers or gateways can have much more complex rules for routing that allow exceptions to this example.

Please be aware that in order to be forwarded, data packets must be addressed to a router. They cannot just be sent to the recipient's address out to a network. The router does not pick packets off the network and forward them. If a packet is sent on a network and a valid recipient is not on that network, there will be no response. This will be demonstrated in the next section where a subnetwork will be described.

To keep routing simple, most networks are structured as shown below. Generally, the higher networks are 10.x.x.x, then the next are 10.0-254.x.x, then 10.0-254.0-254.x. The number 10 is used as an example Class A network. This numbering scheme keeps routing simple and is the least confusing but networks can be set up in other ways. In the diagram below, only gateways and their networks are shown.

In my simple network example below I vary from convention and make network 192.168.2.x be below network 192.168.1.x. causing traffic between the internet and 192.168.2.x to go through the network 192.168.1.x. Normally the network 192.168.1.x would be 192.168.x.x, but this will show you that there can be many variants that will work as long as you have thought your layout through well, and set your routing tables up in your gateways correctly.

The boxes labeled A and B must be gateways or routers in order for anyone on networks 192.168.2.x or 192.168.1.x to talk to any other network or internet. The boxes labeled S1 through S6 are stations which could be workstations or servers providing services like BOOTP, DHCP, DNS, HTTP, and/or file sharing such as NFS or Samba. The gateways may also provide these services. These stations may combine any combination of server or workstation function. The reasons for putting the various services on separate machines is because of security concerns and the ability of a given machine to handle specific demand. Typically, the computer that is connected directly to the internet, would be a firewall and provide no other services for security reasons. For example, it is not a good idea to provide TFTP services on a machine that you want to have high security. This is why, depending on the security needs of the company or individual along with the relative amount of each service to be provided, various servers are set up with limited functionality.

The machine S6 in the diagram above has the following characteristics:

IP Address:   192.168.2.2
Network:      192.168.2.0
Netmask:      255.255.255.0
Gateway:      192.168.2.1

In Linux, the "ifconfig" command is used to configure the NIC and the command "route" is used to set up routing tables for that machine. Please note that in Redhat Linux, the GUI interface programs "netconf" and "linuxconf" may be used to set this up also. These GUI interface programs will set these changes up to be permanent by writing them to files that are used to configure network information. Changes made with "route" without adding the changes to permanent files will no longer be valid when you reboot the machine. The command "ifconfig eth0 192.168.2.2 netmask 255.255.255.0" will set the NIC card up with its address and network number. You can type "netconfig", then select "basic host information" and do the same thing. The command "route add -net default gw 192.168.2.1 dev eth0" will add the route required for this computer for its gateway. This can be done using "ifconf" by selecting "routing and gateways" and "defaults", then setting the address of the default gateway, and enabling routing. Please be aware that various versions of Linux have different means of storing and retrieving network and routing information and you must use the tools that come with your system or learn it well enough to determine what files to modify. On Redhat 6.1 the file "/etc/sysconfig/static-routes" can be modified to make your route changes permanent, but this does not apply to your default route. Other files are "/etc/sysconfig/routed" and "/etc/sysconfig/network". Other files include "/etc/gateways", "/etc/networks", "/proc/net/route", "/proc/net/rt_cache", and "/proc/net/ipv6_route". The file "/etc/sysconfig/network-scripts" is a script file that controls the network setup when the system is booted.

If you type "route" for this machine, the routing table below will be displayed:

Destination	Gateway	Genmask	Flags	Metric	Ref	Use	Iface
192.168.2.2	*	255.255.255.255	UH	0	0	0	eth0
192.168.2.0	*	255.255.255.0	U	0	0	0	eth0
127.0.0.0	*	255.0.0.0	U	0	0	0	lo
default	192.168.2.1	0.0.0.0	UG	0	0	0	eth0

Here is a simple explanation of routing tables and their purpose. All computers that are networked have a routing table in one form or another. A routing table is a simple set of rules that tell what will be done with network packets. In programming language it is easiest to think of it as a set of instructions, very similar to a case statement which has a "default" at its end. If can also be thought of as a series of if..then..elseif..then..else statements. If the lines above are labeled A through C and a default (the last line), an appropriate case statement is: (Don't count the header line)

switch(address){
   case A: send to me;break;
   case B: send to my network;break;
   case C: send to my local interface;break;
   default: send to gateway 192.168.2.1

An appropriate if statement is:

if (address=me) then send to me;
elseif (address=my network) then send to my network;
elseif (address=my local) then send to my local interface;
else send to my gateway 192.168.2.1;

In everyday terms this is similar to a basic decision process. Imagine you are holding a letter. If it is addressed to you, you keep it, if it is addressed to someone in your town, you drop it in the local slot at the post office, but if it is addressed to someone out of town, you would drop it in the out of town slot.

Note how the routing table is arranged. It is arranged from the most specific to the least specific. Therefore as you go down the table, more possibilities are covered. You will notice the first Genmask is 255.255.255.255 and the last is 0.0.0.0. There can be no doubt that the last line is the default. The genmasks between the start and the end have a decreasing number of least significant bits set.

The above default routing table may be added manually with the command:

route add -net default gw 192.168.2.1 dev eth0

The routing table for machine B, the gateway for the network 192.168.2.0 is as follows.

Destination	Gateway	Genmask	Flags	Metric	Ref	Use	Iface
192.168.2.1	*	255.255.255.255	UH	0	0	0	eth0
192.168.1.2	*	255.255.255.255	UH	0	0	0	eth1
192.168.2.0	192.168.2.1	255.255.255.0	UG	0	0	0	eth0
192.168.2.0	*	255.255.255.0	U	0	0	0	eth0
192.168.1.0	192.168.1.2	255.255.255.0	UG	0	0	0	eth1
192.168.1.0	*	255.255.255.0	U	0	0	0	eth1
127.0.0.0	*	255.0.0.0	U	0	0	0	lo
default	192.168.1.1	0.0.0.0	UG	0	0	0	eth0

The Iface specifies the card where packets for this route will be sent. The address of eth1 is 192.168.1.2 and eth0 is 192.168.2.1. The NIC card addresses could have easily been switched. Line 1 (above) provides for the eth0 address, while line 2 provides for the address of eth1. Lines 3 and 4 are the rules for traffic going from network 192.168.1.0 to network 192.168.2.0 which will be sent out on NIC eth0. Lines 5 and 6 are the rules for traffic going from network 192.168.2.0 to network 192.168.1.0 which will be sent out NIC eth1. This may seem confusing, but please note the first value on lines 3 and 4 is 192.168.2.0 which the header indicates as the destination of the packet. Don't think of it as source! The last line is the default line which specifies that any packet not on one of the networks 192.168.1.0 or 192.168.2.0 will be sent to the gateway 192.168.1.1. This is how the internet access can be attained, though IP masquerading will probably be used. The flags above mean the following:

• U - Route is up
• H - Target is a host
• G - Use gateway

There are other flags, you can look up by typing "man route". Also the metric value above, indicating the distance to the target, is not used by current Linux kernels but may be needed by some routing daemons. Please note that if route knows the name of the gateway machine, it may list its name rather than the IP address. The same is true for defined networks. Networks may be defined in the file "/etc/networks" as in the example:

net1 192.168.1.0
net2 192.168.2.0

The routing table above can be set up with the following commands.

route add -net 192.168.2.0 netmask 255.255.255.0 gw 192.168.2.1 dev eth0
route add -net 192.168.1.0 netmask 255.255.255.0 gw 192.168.1.2 dev eth1

Again be aware that you are specifying destination networks here and the ethernet device and address the data is to be sent on.
In Redhat Linux this can be specified using "netconf" by selecting "routing and gateways" and "other routes to networks" and entering the following:

Network	Netmask	Gateway
192.168.2.0	255.255.255.0	192.168.2.1
192.168.1.0	255.255.255.0	192.168.1.2

Alternatively in Redhat Linux, you can add the following two lines to the file "/etc/sysconfig/static-routes":

eth0 net 192.168.2.0 netmask 255.255.255.0 gw 192.168.2.1
eth1 net 192.168.1.0 netmask 255.255.255.0 gw 192.168.1.2

The commands to delete the above routes with route are:

route del -net 192.168.2.0 netmask 255.255.255.0 gw 192.168.2.1 dev eth0 route del -net 192.168.1.0 netmask 255.255.255.0 gw 192.168.1.2 dev eth1

Be aware, the program route is very particular on how the commands are entered. Even though it may seem that you entered them as the man page specifies, it will not always accept the commands. I don't know if this is a bug or not, but if you enter them as described here with the network, netmask, gateway, and device specified, it should work. The slightest misnomer in network name, netmask, gateway, device, or command syntax and the effort will fail.

How to Delete a Windows Service in Vista or XP

If you are a fan of tweaking your system and disabling services, you might find that over time your Windows Services list becomes huge and unwieldy with a large number of services in the list that will never be enabled.

Instead of just disabling a service, you can alternatively completely delete the service. This technique can be especially helpful if you've installed some piece of software that doesn't uninstall correctly, and leaves an item in the service list.

Important Note: Once you delete a service, it's gone, and it's going to be a pain to add it back. Use with caution.

Deleting a Service

The first thing you'll need to do is identify the name of the service, so open up Services through the start menu or control panel, and then find the service in the list that you want to delete.

You'll want to open up the properties by double-clicking on the service name, and then highlight the "Service name" value and copy it to the clipboard. This is what we'll need to disable it.

You'll need to open up a command prompt, and if you are using Windows Vista you'll need to right-click the command prompt and choose Run as Administrator. We'll use the sc command to actually do the work.

The syntax used to delete a service is this:

sc delete ServiceName

If your service name has spaces in it, you'll need to wrap the service name in quotes, like this:

sc delete "Adobe LM Service"

Note that I'm not recommending deleting this particular service, it's just an example.

Now if you use the F5 key to refresh your Services list, you'll see that the service is gone.

I've found that using this technique (carefully) can make your Services list a lot more useful, since you don't have to weed through dozens of items you will never have enabled.

Note: You should think long and hard before deleting a service, because it's very difficult to get them back once they are gone.

Advanced iTunes 8 Management

Here's how to get a better handle on that overloaded, multi-computer, multi-gadget music library of yours.

Inexpensive iPods, music-playing cell phones, and—heck—even PCs have essentially replaced the traditional stereo system. As the typical stack of 17-inch-wide audio components goes the way of the dodo, so too do their accompanying fiberboard shelving units stuffed with hundreds of plastic CD jewel cases.

That doesn't mean that your organizational chores are over, unfortunately. The average music fan may have one or more PCs, an MP3 player, a cell phone, a car stereo, and other music-playing gadgets. And while you won't catch many of us returning to the days of physical media for music—despite what SanDisk may think—it's still tough to keep everything under control. With that in mind, power up your iTunes 8 organization, navigation, and maintenance skills with the following tips:

(Note: If you're using a Mac, typically replacing the Ctrl key with the Command key will yield the same results.)

Improve your playlists. Here are some ways to speed up playlist management:

1. Organize your playlists in folders by clicking File | New Playlist Folder, and then dragging playlists into each one.


2. Create a playlist of an entire album by clicking Music in the Library menu at left, and then View | Show Browser. Then drag the album from the Album list to the space beneath everything else in the big bluish pane at left (minimize the playlists item if you have to)—or just Shift-click the Add (+) button on the album name to do the same thing.
3. Create an instant playlist of a group of selected songs by Ctrl-clicking each song, and then dragging the whole batch to the same space on the left.
4. Right-click a song name to see a bunch of useful shortcuts, including the ability to display all the playlists in which a song currently appears (Show in Playlist), or cue it as the next song in Shuffle mode.
5. Mac only:
   Delete a song from a playlist (as well as from all other playlists and your library) in just one step by pressing Option-Delete.

Navigate iTunes like a pro.

1. Ever lose your place in the Library view while listening to a song? Jump to the currently playing track by clicking the curved arrow to the right of the progress bar at the top.


2. Right-click any column heading in order to change which ones are visible.
3. Work the iTunes controls from the keyboard by holding down the Ctrl key: Ctrl-Right Arrow skips to the next song, while Ctrl-Left Arrow skips back. Ctrl-Up and Ctrl-Down change the volume, while the space bar plays or pauses the current track.

Delete duplicate songs. Now that Apple is finally pulling DRM off its iTunes Store tracks, 30-cent upgrades are available for prior purchases, and, thrown into the bargain, the upgrades double the bit rate—to much-better-sounding 256-kilobit-per-second AAC files. Although iTunes offers to delete the old 128-Kbps files in the process, I found that iTunes misses a lot of them—seemingly because the iTunes Store changed the name of albums along the way. So knock 'em all out in one fell swoop by selecting File | Show Duplicates, and then deleting all the unwanted versions.

Fix incorrect album art. Sometimes iTunes goofs when downloading album artwork. Erase the incorrect images by highlighting all the appropriate tracks, right-click a song title, and select Clear Downloaded Artwork. Then manually drag an appropriate image over to the album art box. Or let iTunes do another pass by clicking Advanced | Get Album Artwork, which then checks the art on all your tunes.

Beef up artist recommendations. iTunes 8's new Genius feature is pretty cool, but take it a step further and install the free iLike Sidebar. It attaches to the right of your iTunes window and generates new recommendations, free MP3 links, and even related videos as you play different tracks.

Create extra libraries. Apple wants you to keep all of your music in one library, but that doesn't mean you have to. Hold down the Shift key when starting up the app; you'll see a small dialog box asking whether you want to create a new library or look for an existing one. This little trick lets you archive music you don't feel like staring at all the time but don't want to delete forever, either—and it's also great for people with different musical tastes who share the same computer.

Improve the sound quality of imported CDs. Now that storage prices have fallen through the floor, and the standard iPod classic has a 120GB hard disk, consider bumping up the sound quality of imported CD tracks to at least 256 Kbps AAC, if not Apple Lossless. If you've already imported all your CDs, you could—gulp—do it again at a higher bit rate or losslessly as well. (That's what I ended up doing.)

BONUS TIP: Make it fun. There's no need to stare at a featureless backdrop of song and album titles all day. Don't forget that iTunes lets you spruce up the interface with a couple of built-in extras. The first is the app's beautiful Cover Flow feature, which turns your PC into a virtual jukebox—click the right-most icon under View at the top right of the window to activate it. There's also the customizable Visualizer, which ebbs and flows in time to the music; toggle it by pressing Ctrl-T.