This is an Internet Protocol (IPv4) Subnet Chart. You can use this to quickly look up how you might need to subnet your network. At the bottom there is a quick how-to on calculating subnets.

For more information on subnetting, see RFC 1817 and RFC 1812.

Class address ranges:

Class A = 1.0.0.0 to 126.0.0.0

Class B = 128.0.0.0 to 191.255.0.0

Class C = 192.0.1.0 to 223.255.255.0

Reserved address ranges for private (non-routed) use (see RFC 1918):

10.0.0.0 -> 10.255.255.255

172.16.0.0 -> 172.31.255.255

192.168.0.0 -> 192.168.255.255

Other reserved addresses:

127.0.0.0 is reserved for loopback and IPC on the local host

224.0.0.0 -> 239.255.255.255 is reserved for multicast addresses

Chart notes:

Number of Subnets - "( )" Refers to the number of effective subnets, since the use of subnet numbers of all 0s or all 1s is highly frowned upon and RFC non-compliant.

Number of Hosts - Refers to the number of effective hosts, excluding the network and broadcast address.

Class A

Computer networking professionals getting started with Packet Tracer may find the interface to be flustered. Being a development program, this is only natural. However, learning how to configure a router with Packet Tracer will put professionals on the right track to mastering the program in about half an hour.

By this time, you should already have the Packet Tracer download and have it installed on your computer. Open the program and select the router from the lower left-hand corner, and drag it into the center of the sandbox screen as seen below. (Click for larger picture)

The Transport layer- is the layer responsible for actually getting the data packets to a specific location. When we receive email, we want to open it with our email program- not anything else. So how does a computer know exactly where to route data to appropriate programs, all while dealing with multiple connections?

The Difference between TCP and UDP

We mentioned earlier that a transport protocol can have simultaneous connections to a computer- yet the receiving computer still knows where each data packet should be sent. This is accomplished through ports and sockets.

A port is simply an internal address that acts as a pathway to control data flow. Since we need each port to be specific to a certain application, there are thousands of ports for use. If you are using the internet, for instance, data is being routed through TCP port 80. This port is called the HTTP port. Ports that have specific purposes (such as the HTTP port) are also known as well-known ports. (And in case you were wondering, there are over 65,535 ports to experiment with.)

 So now we know two things. First, the IP address is used to route the data to a specific computer. Next, the port number is used to tell the receiving computer what kind of application should handle it. But to actually accomplish both of these tasks, we use what is called a socket. A socket is simply an address formed by using the IP address and then tacking on the port number at the end.

Practical Uses of the Socket

So what good is a socket? For one thing, multiplexing and demultiplexing is made possible. When you are running multiple programs that are communicating with other computers, you are making use of both of these technologies.

Let’s look at a practical example. You are running a telnet server in which multiple computers are connected to. Each computer uses a socket address to tell the server which computer and which port the data is coming from. If each computer broadcasted at the same time, there may be a jam at the transport layer. We use multiplexing in this case to combine all incoming data into one stream.

Demultiplexing is very similar to multiplexing, except that it works in reverse. Instead of taking multiple streams of input data and outputting a single stream of data, demultiplexing involves receiving a single stream of input and delivering it to multiple outputs. This is handy for separating multiplexed signals. You may see demultiplexing referred to as “Demuxing.”

Don’t worry- UDP is a lot simpler as compared to the TCP counterpart. This is due to the lack of error checking UDP contains. Although UDP is famously known for not having error checking, there are slight UDP functionalities that can act as basic error checking procedures.

It’s important to know that the UDP error checking functionalities are quite basic. For instance, the UDP protocol contains functionality for a checksum so that we may check the integrity of data. This field can be turned off, however, so that a faster connection can be had. Also important to note is that UDP uses a fake header (known as a pseudo-header) that contains a destination address, so that we can determine whether or not the packets were sent to the right place. If they are indeed sent to the wrong place, a simple ICMP message may notify the source machine that the source is unreachable.

Keep in mind that even if errors are found within a UDP connection, data will not be retransmitted. So how much different is a UDP datagram from a TCP segment?

UDP Datagrams Explained

1. Source Port - An optional 16-bit field that specifies which port the datagram originated from.

The Collision Detection and Solution Process

If you ever lose a password on a PIX Firewall and need to recover it, follow these steps:

if you ever need to recover a password on the PIX
here are the steps:

Requirements:

* You will need a console connection from the PIX to your machine
* You will need a TFTP server application running on your machine
(I personally use tftpd32, which can be downloaded for free
at http://tftpd32.jounin.net/)

1) Connect the console cable to your serial port and plug the RJ45 end
into the PIX port marked "Console". You can use Hyperterm (which comes
with Windows) or any other console program of your choice. I use
TeraTerm Pro, found at http://www.ayera.com/teraterm/.

2) Find out what version of software is running on your PIX. If you're
not sure, you can find out very easy in the following way. If you
are connected to the PIX via a console connection, simply reboot the
PIX and watch for the output. It will tell you which version is
running.

3) Download the corresponding helper binary file from Cisco, depending
on which software version is running on the PIX. For example, if you
were running version 6.3(x) you could use the file called np63.bin found
here: http://www.cisco.com/warp/public/11... If you were running
6.2 you could simply change the last characters on the above url to be
np62.bin. Download that file and save it to the root directory of your
TFTP application.

4) Next, reboot the PIX again and immediately after the reboot as it is
coming back up and displaying text in your console send a break sequence
with your keyboard. If you are using Hyperterminal with Windows the break
sequence is Ctrl-Break.

5) This will send the PIX into "Monitor" status and you will see the following
prompt on the PIX:

monitor>

6) Patch your computer into the inside or outside interface on the PIX via a
standard CAT 5 cable (i.e. patch from your computers NIC to one of the PIX's
interfaces).

7) Give your computer an IP address. For this example, let's use 10.0.0.1
with a gateway of 255.0.0.0

8) Start up your TFTP server program and keep it running.

9) Tell the PIX which interface you will be connecting to, as follows:

monitor> interface 1

*note interface 1 is inside, interface 0 is outside, but you remember that
from reading the ebook right? :)

10) Give the PIX a temporary IP address on the same network as your computer,
as follows:

monitor> address 10.0.0.2

11) Tell the PIX the IP address of the TFTP server (your computer)

monitor> server 10.0.0.1

12) Tell the PIX which file to copy:

monitor> file np63.bin

13) Start the TFTP copy

monitor> tftp

14) It should copy very quickly. If it does not you will get
an error message on the PIX and potentially on the TFTP server
software. If you do get an error, you likely have a cabling
issue or perhaps a typo of one of the above commmands.

15) Once the file is copied to the PIX, the PIX will ask if you are
sure you want to reset the password. Type "Y" for yes, and the PIX
will reboot.

16) After the reboot the PIX will now have a default telnet password
of "cisco" (no quotes) and no enable password.

That's about it. About 10 minutes of downtime and you and your PIX Firewall are back in action!

A brief guide on how to properly add a second (or third) router to an existing LAN that already has a SOHO router connected to the internet.

The general setup of SOHO Router's is similar.  While some use a different subnet, every one I've worked with uses a Class C Private IP Addressing Scheme.  I'm going to use the basic Class C private for this tutorial (192.168.0.0)

I'm going to give two setup guides:

Because ISPs don't assign static IP addresses, accessing devices your network remotely is tricky. One way around changing IP addresses is to use a Dynamic DNS service, which automatically tracks the changes to your network's public IP address.

You pobably use your broadband connections for all kinds of things when you're at home, but you can also take advantage of that connection to access devices on your network even while you're away. For example, you might want to view a Webcam, grab files off an FTP server, use a remote access utility like Windows' Remote Desktop, or even access the router itself to administer it from afar.

These sorts of things would normally be made possible by a router's port forwarding feature combined with knowing the public, or global, IP address assigned to your network by your ISP. The problem is that for most accounts, that public address isn't permanent and will change from time to time. It may not change very often — maybe every few days or perhaps even less frequently than that — but once it does, the address you've become accustomed to using won't work anymore. While you can always find the new IP address and begin using it instead, you can't look it up while away from home. Even if you could, the address will eventually change again. 

Static or Dynamic

One way to eliminate this problem is to arrange for a static IP address through your ISP, but this isn't always feasible. For starters, getting one can cost $5 (or much more) a month. With some ISPs, a static IP option isn't available unless you upgrade to a more expensive business-class account (and that pricier service may not even offer any more bandwidth than you already have).

A better solution is to use a Dynamic DNS service that can automatically track the changes to your network's public IP address. These services let you set up your own specific DNS name and then will regularly update a database to make sure the name always points to your current IP address.

Configuring Dynamic DNS

The most common way to use Dynamic DNS is to configure your broadband router to automatically update the service each time it gets a new IP from your ISP. There are several Dynamic DNS services to choose from, most of which are free or available at a nominal cost. The two most popular are DynDNS and TZO. While both services are excellent, we'll focus on DynDNS here because it offers a free version and arguably has the most widespread router support. Almost every router offers some kind of Dynamic DNS support, but your router must support the particular service you want to use. You can access the Dynamic DNS heading in your router's admin interface to see which services it will work with (and you may be presented with additional options if you upgrade to the latest firmware).

Prior to making any router modifications, start here by creating a DynDNS. You'll need to confirm your account via e-mail before you can configure it — once you've done that, look for an Add Host link or go here.

In the hostname field, you can enter anything that means something to you, assuming it's hasn't already been used (let's say joesnetwork, for example). Then pick the domain name you want to use from the rather extensive pull-down list (there are 68 choices in all), make sure your IP address is filled in (it should have been done automatically), and click Add Host.

Now call up the Dynamic DNS configuration page of your router (the exact interface will vary depending on the make and model of your hardware). First ensure Dynamic DNS is enabled, specify the service provider you're using, and then enter your DynDNS user name, password, and the full host name plus domain name you choose (e.g. joesnetwork.dyndns.org). Be sure to apply the changes; a router reboot will probably be necessary.

Remember Port Forwarding and Internal Static Ips

Just as you would if accessing your network via an IP address, before you start using Dynamic DNS you should make sure your router's port-forwarding feature is configured appropriately so the router knows which IP address to forward incoming traffic to. For example, if you're setting up a Web server, forward port 80; an FTP server, port 21; or Remote Desktop, port 3389.

It's also critical that you assign a static internal IP address to any device you're trying to reach so that the router always forwards to the right place. Another way to accomplish this is to use the DHCP reservation feature found on many routers, which will make sure a given device always gets the same DHCP-assigned address.

Using Your New Domain Name

It may take up to an hour after you set up your DynDNS domain name before it's usable. Once active, all you need to do is use the domain name instead of the IP address with the relevant software. The nice thing about Dynamic DNS combined with port forwarding is that you can use the same name to access multiple systems on your network. As long as you've set up the port forwarding correctly, the type of software you use or port you specify will determine what you connect to.

Although the domain names that DynDNS offers aren't particularly memorable, they're probably sufficient as long as you're the only one that needs to know them. If you plan to allow others access to your network and want to use a more meaningful name, TZO offers a Dynamic DNS service that will let you choose a custom domain name for about $60 a year.

A final caveat: Be sure to check your ISP's terms of service before setting up a server that's going to be public and generate lots of traffic (especially a Web server), because some ISP rules prohibit customers from running such servers (particularly with consumer-grade accounts). It usually won't be an issue when you're setting up personal access, but in some cases ISPs may filter incoming traffic or terminate your account if it detects an unauthorized server. (Another of TZO's paid services provides a forwarding feature that will let you run a web server even if your ISP blocks port 80.)

There you have it. We've only scratched the surface of what Dynamic DNS can do, but the most important thing to remember is that it will ensure that your network is always locatable no matter how often your public IP address changes.

This is a cool batch script that I found to ping a range of IP addresses and report whether they respond to the ping.

Simply copy and paste this code into a notepad file and save with the bat extension (remember to enclose the save as file name in quotes so that it doesnt add a txt extension.

Today I'd like to share with you some light ideas about network troubleshooting in Windows XP.
Most of your network connection issues can be fixed with an very enigmatic command called “Repair”. You can find this by clicking right mouse button on network icon at System Tray area. Please take a look on a picture below.

PsExec

psexec \\RemoteComputer cmd.exe

SSH on Windows

A common tool that network and system admins make use of is the “Ping” command which is a very simple and effective way to verify a machine is available on the network (firewall rules depending of course). So if you find yourself having to ping multiple machines at once, a very useful tool is the batch files, which will not only ping all the ip addresses but also give you ping results in text file format with just single click.

A batch file is a text file containing a series of commands intended to be executed by the command interpreter. When a batch file is run, the shell program (usually COMMAND.COM  or cmd.exe) reads the file and executes its commands, normally line-by-line. Batch files are useful for running a sequence of executables automatically and are often used by system administrators to automate tedious processes.

Steps to create batch file::

1. Open up Notepad, type in the following commands (Example)