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 A Brief Introduction Into TCP/IP

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PostSubject: A Brief Introduction Into TCP/IP   Mon Nov 17, 2008 4:29 pm

Many people may not know what TCP/IP is nor what its effect is on the
Internet. The fact is, without TCP/IP there would be no Internet. And
it is because of the American military that the Internet exists.During
the days of the cold war, the defense department was interested in
developing a means of electronic communication which could survive an
attack by being able to re-route itself around any failed section of
the network.They began a research project designed to connect many
different networks, and many different types of hardware from various
vendors. Thus was the birth of the Internet (sorta). In reality, they
were forced to connect different types of hardware from various vendors
because the different branches of the military used different hardware.
Some used IBM, while others used Unisys or DEC.

TCP
(Transmission Control Protocol) and IP (Internet Protocol) were the
protocols they developed. The first Internet was a success because it
delivered a few basic services that everyone needed: file transfer,
electronic mail, and remote login to name a few. A user could also use
the “internet” across a very large number of client and server systems.

As with other communications protocols, TCP/IP is composed of layers. Each layer has it’s own responsibility:

IP
is responsible for moving data from computer to computer. IP forwards
each packet based on a four-byte destination address (the IP number).
IP uses gateways to help move data from point “a” to point “b”. Early
gateways were responsible for finding routes for IP to follow.

TCP
is responsible for ensuring correct delivery of data from computer to
computer. Because data can be lost in the network, TCP adds support to
detect errors or lost data and to trigger retransmission until the data
is correctly and completely received.

How TCP/IP works

Computers
are first connected to their Local Area Network (LAN). TCP/IP shares
the LAN with other systems such as file servers, web servers and so on.
The hardware connects via a network connection that has it’s own hard
coded unique address – called a MAC (Media Access Control) address. The
client is either assigned an address, or requests one from a server.
Once the client has an address they can communicate, via IP, to the
other clients on the network. As mentioned above, IP is used to send
the data, while TCP verifies that it is sent correctly.

When a
client wishes to connect to another computer outside the LAN, they
generally go through a computer called a Gateway (mentioned above). The
gateway’s job is to find and store routes to destinations. It does this
through a series of broadcast messages sent to other gateways and
servers nearest to it. They in turn could broadcast for a route. This
procedure continues until a computer somewhere says “Oh yeah, I know
how to get there.” This information is then relayed to the first
gateway that now has a route the client can use.

How does the system know the data is correct?

As mentioned above, IP is responsible for getting the data there. TCP then takes over to verify it.

Encoded
in the data packets is other data that is used to verify the packet.
This data (a checksum, or mathematical representation of the packet) is
confirmed by TCP and a confirmation is sent back to the sender.

This process of sending, receiving and acknowledging happens for each individual packet sent over the Internet.

When
the data is verified, it is reassembled on the receiving computer. If a
package is not verified, the sending computer will re-send it and wait
for confirmation. This way both computers – both sending and receiving
– know which data is correct and which isn’t.

One nice thing
about this protocol is that it doesn’t need to stick to just one route.
Generally, when you are sending or receiving data it is taking multiple
routes to get to its destination. This ensures data accuracy.

Just the facts:

TCP/IP
addresses are based on 4 octets of 8 bits each. Each octet represents a
number between 0 and 255. So an IP address looks like: 111.222.333.444.

There are 3 classes of IP addresses:

Ranges starting with “1” and ending with “126” (i.e.. 1.1.1.1 to 126.255.255.254) are Class A

Ranges starting with “128” and ending with 191 (i.e.. 128.1.1.1 to 191.255.255.254) are Class B

Ranges
starting with 192 and ending with 254 (i.e.. 192.1.1.1 to
254.255.255.254) are Class C ( You will notice that there are no IP
addresses starting with “127”. These are reserved addresses.)

Calculating an IP address

One
of the things that always confused me was how to convert IP address to
their Binary form. It is quite simple really. IP addresses use the
Binary numbers (“1”s and “0”s) and are read from right to left.

Each position in the binary address corresponds to a number, from 1 to 128 and look like this:

128 64 32 16 8 4 2 1

To calculate an address, simply add the numbers where a “1” appears.

For example, the following:

00001010 works out to 10. Like this:

0 0 0 0 1 0 1 0
128 64 32 16 8 4 2 1

You can see that the “1”s line up with the 2 and 8 – when you add 2 plus 8 the answer is 10.

Since an IP address contains 4 of these octets, it can be displayed in binary like:

00001010.00001010.00001010.00001010

Therefore, IP Address 10.129.254.1 would be converted to:

00001010.10000001.11111110.00000001
(8+2) . (128+1) .(128+64+32+8+4+2).(1)

While
it’s not important for the average person to know how to figure this
stuff out, it is important for someone setting up a small network. That
is because TCP/IP also uses what are called subnet masks to determine
which addresses are valid.

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