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TCP Source & Destination Port Number - Section 1

Posted in TCP Protocol Analysis

This section contains one of the most well-known fields in the TCP header, the Source and Destination port numbers. These fields are used to specify the application or services offered on local or remote hosts. We explain the importance and functionality of the TCP source and destination ports, alongside with plenty of examples.

You will come to understand how important ports are and how they can be used to gain information on remote systems that have been targetted for attacks. We will cover basic and advanced port communications using detailed examples and colourful diagrams, but for now, we will start with some basics to help break down the topic and allow us to smoothly progress in to more advanced and complex information.



When a host needs to generate a request or send data, it requires some information:

1) IP Address of the desired host to which it wants to send the data or request.

2) Port number to which the data or request should be sent to on the remote host. In the case of a request, it allows the sender to specify the service it is intending to use. We will analyse this soon.

1) The IP Address is used to uniquely identify the desired host we need to contact. This information is not shown in the above packet because it exists in the IP header section located right above the TCP header we are analysing. If we were to expand the IP header, we would (certainly) find the source and destination IP Address fields in there.

2) The 2nd important aspect, the port number, allows us to identify the service or application our data or request must be sent to, as we have previously stated. When a host, whether it be a simple computer or a dedicated server, offers various services such as http, ftp, telnet, all clients connecting to it must use a port number to choose which particular service they would like to use.

The best way to understand the concept is through examples and there are plenty of them below, so let's take a look at a few, starting from a simple one and then moving towards something slightly more complicated.


Time To Dive Deeper!

TCP Sequence & Acknowledgement Numbers - Section 2

Posted in TCP Protocol Analysis

This page will closely examine the Sequence and Acknowledgement numbers. The very purpose of their existence is related directly to the fact that the Internet, and generally most networks, are packet switched (we will explain shortly) and because we nearly always send and receive data that is larger than the maximum transmission unit (a.k.a MTU - analysed on sections 5 and 6 ) which is 1500 on most networks.

Let's take a look at the fields we are about to analyse:


As you can see, the Sequence number proceeds the Acknowledgement number.

We are going to explain how these numbers increment and what they mean, how various operating systems handle them in a different manner and lastly, what way these numbers can become a security hazard for those who require a solid secure network.

Direct Cable Connection

Posted in Network Cabling

network direct cable connection and Data transferFrom the early PC days, Direct Cable Connection (DCC) was the most popular way to transfer data from one PC to another. Of course, it might seem a bit of an "old fashioned" way to transfer data these days but remember that back then most PC's were running Dos 6.22 or Windows for Workgroups 3.11 if you were lucky!

Today, all computers are equipped with a network card and using straight-thru or cross-over network cables, we are able to quickly transfer data at speeds much greater than a serial or parallel cable. However , there are still times where we require a transfer via the serial or parallel port, and that's what this page is about.

Transferring data between computers via direct cable connections can be performed using the following methods:

  • Serial Cross-over cable
  • Parallel cable – also known as ‘LapLink’ cables
  • USB Transfer or Data Link cable
  • UTP Cross-over or Straight-thru cable

Serial Direct Cable Connection, DB9, DB25, COM Ports and Pinouts

Posted in Network Cabling

This article covers the popular serial ports on workstations, servers and laptop computers. We cover serial data transfer, port pinouts, port speeds, serial interface types (DB9 & DB25), null modem cables and much more.

The Serial Direct Connection is the one which utilizes the COM ports of your computers. Every computer has at least two COM ports, COM1 and COM2. The "COM" stands for "Communications". Its pinouts are a lot simpler when compared to the parallel port, but the speed is also a lot slower.

To give you an idea of how fast (or slow) a serial port is, at its best you will get around 12 to 14 KB per second. That's pretty slow when you're used to a network connection, but let me show you how serial data is transferred so you can also understand why it's a lot slower:

Transfer of data via serial port

Figure 1. Transfer of data via serial port


The above picture gives you an idea on how serial data is transferred. Each colored block that is numbered is sent from PC 1 to PC 2. PC 2 will receive the data in the same order it was sent, in other words it will receive data block 1 first and then 2, all the way to block 7. This is a pretty good representation of data flow in a serial cable. Serial ports transmit data sequentially over one pair of wires (the rest of the wires are used to control the transfer).

Another way you can think of it is like a one lane road where the road is wide enough to only fit one car at a time (one data block at a time in our example above) so you would imagine that the road cannot process several cars at one time.

The Serial Port

Most new computers have two COM ports with 9 pins each; these are DB-9 male connectors. Older computers would have one DB-9 male connector and one DB-25 male connector. The 25 pin male connector is pretty much the same as the 9 pin, it's just bigger.

Let's have a look at a serial port to see what we are talking about:

Physical Serial interface - DB-9 (usually COM1) and DB-25 (usually COM2)

Figure 2. Physical Serial interface - DB-9 (usually COM1) and DB-25 (usually COM2)

Different pinouts are used for the DB-9 and DB-25 connectors and we will have a look at them in a moment. Let's just have another quick look at the COM ports of a new computer:


Figure 3. Serial ports (COM1 & COM2)

Notice the COM ports, they are both DB-9 connectors, there is no more DB-25! The connector above the two blue COM ports is an LPT or Parallel port.

LPT Ports - Parallel Direct Cable Connection - Pinouts - Transfer speeds

Posted in Network Cabling

LPT Ports - Parallel Direct Cable Connection - Pinouts - Transfer speedsThe Parallel Direct Connection is the second solution covering the transfer of data from one computer to another. The cable required is slightly more complicated as it has more wires that need to be connected, but the transfer speeds achieved make it well worth the time and effort required to make the cable. We'll also take a look at physical LPT ports, LPT modes (SPP, EPP, ECP), LPT port Pintouts, LPT direct connection cable and more.

Users interested in transferring files using parallel direct cables can visit the following Microsoft support page which explains How to Install and Configure the Direct Cable Connection Feature (https://support.microsoft.com/en-us/kb/298446).

As we’ll see further below, there are three different type of LPT ports, SPP, EPP and ECP, each supporting different speeds and features, but all use the same direct cable connection.

A standard LPT port will provide speeds of 40Kb/s to 60Kb/s while the faster ECP ports will deliver up to 1.1 Mb/sec or 8.8 Mbps.

To better understand why parallel links are much faster than serial links, we’ll need to analyze the way data is transferred. This is clearly shown and explained in the diagram below:

Transfer of Data via Parallel (LPT) Port

Figure 1. Transfer of Data via Parallel (LPT) Port

This diagram shows data transfer via parallel ports and we can see multiple data blocks being simultaneously transferred from one host to another, increasing significantly the overall throughput. Serial ports are capable of transferring one data block per time, therefore unable to match speeds of parallel ports.

What does the parallel port (LPT) look like?


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