TCP/IP Primer

TCP/IP is a subject that anyone interested in networking needs to have a basic understanding.  Being able to create a small network and make all of the computers and other devices communicate with each other can be tricky.  With just a little understanding of the basics, the job becomes easy.

First, lets take a look at a few terms used in networking.  Any device attached to a network that has an IP address is known as a "node" or a "host."  An IP address consists of four octets separated by periods.  For example 10.1.1.5 is a basic IP address.  The octets are so called because they are actually decimal representations of binary numbers.  The number 10 for instance, when broken into binary, becomes 00001000.  Note that it is eight bits long and equals one byte of information.  Each octet can be broken down the same way.  To understand binary, refer to the chart below.  The numbers across the top of the chart show the places in the binary number, the numbers down the left side are decimal numbers.  The 1's and zero's show which bits are used to make up the decimal number.

By adding the value (top row) of each row with a 1 in the cell you can easily convert from binary to decimal and vice versa. For instance the number 5 is made up of a 1 in the 4's place and a 1 in the 1's place.  When writing binary numbers it is proper to maintain the unused places with zero's.  Hence the number 5 is represented thusly, 00000101.

In order for two nodes on a network to communicate directly without the aid of a router, they must be in the same "subnet."  This requires us to know the "subnet mask" assigned to the network.  The subnet mask tells us how much of the IP address is the network ID, and how much is the host ID.  For instance, if we have two IP addresses using a subnet mask of 255.255.255.0, the first three octets of the IP address represents the network ID and only the last octet identifies the individual host.  If we have 10.1.1.5 and 10.1.1.254, these two addresses are in the same subnet because the first three octets are the same and only the last octet is different.  The nodes with these addresses assigned to them are able to communicate with each other directly without the aid of a router.  If on the other hand we had 10.1.1.135 and 10.1.2.1, these addresses are not in the same subnet because the third octet is different and two nodes assigned these addresses would not be able to communicate with each other directly.

A word about NAT, Network Address Translation.  It is possible to use an IP address scheme of our own making on our Local Area Network while still being able to use the Internet which uses a wildly different IP addressing scheme.  We use a hardware device called a NAT Router.  A good example of these used in home networks is the Linksys DSL/Cable router with a  four-port built-in switch.  The router simply has two sides, the side that your Internet service provider sees and the side which your computers on your home network see.  Lets say that your ISP assigns you an IP address of 24.158.232.100 with a subnet mask of 255.255.255.0.  You assign that address to your router and connect it to the Internet.  This address becomes the identity of the port on the router labeled "WAN".  So far so good, we have one node connected to the Internet but we have ten computers that need to access the Internet.  You don't need any more Internet IP addresses because you have a NAT router connected to your cable modem.  Your router has another side to it called the LAN ports.  These are the four ports to whick you can connect computers, switches or hubs.  By default Linksys uses an IP address of 192.168.1.1with a subnet mask of 255.255.255.0 for the router and you can use all of the addresses in the 192.168.1 range with the exception of 192.168.1.255.  This address is reserved for broadcast and cannot be assigned to any node.  You can attach up to 253 nodes using from 192.168.1.2 to 192.168.1.254.

In order to put your computer in the same subnet as another device you need to set up IP addresses that match the criteria mentioned above.  If you needed to change the IP settings on your computer to match a router or wireless access point then you can use the following steps:

In Windows 98 right click on the desktop icon for "network neighborhood" and choose properties.  Then you need to find the IP settings for the NIC installed in your computer.  Click on it and then click the properties button.

In Windows 2000 or XP, right click on "my network places" and choose the properties.  Then right click on the local area connection and choose properities again.

Once you have the IP properties open, you will see a box like this:

Now choose the IP and click properties button, you will see this box:

Give your computer an IP address and subnet mask, the default gateway is the address of the device that is connecting you to the Internet.  If you are using a DSL/Cable router then it will be the IP address assigned the that device.  Linksys uses the address of 192.168.1.1 by default on their routers.  Save your settings and restart your computer.  You will now be able to communicate with the other device.

I hope this gives you a little bit of insight into the world of TCP/IP.  Good luck with your networking adventures!

Fred

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