TCP/IP architecture model

1.    Network interface(Data link) layer
2.    Network layer
3.    Transport layer
4.    Application layer

Network interface layer
The lowest layer of the TCP/IP model. Its task is to provide access to the transmission physical medium and it differs according to the implementation of the medium.

Network layer
The network layer provides network addressing, routing and datagram transmission.  Used protocols that will be of interest further regarding DHCP are IP and ARP.

IP protocol
It is the basic protocol of the network layer and in general the internet as a whole. It sends datagrams, which are independent units that contain information about the destination, source and the sequence number of the datagram. The sequence number is used for message reconstruction, since the delivery order of the datagrams might not be the same as their order in the message and delivery reliability isn’t guaranteed at all.
IP protocol versions:
“    IP v4 – 32 bit addresses. Provides approximately 4 billion unique addresses which aren’t sufficient at present times.
“    IP v6 – 128 bit addresses. The transition to v6 will bring (is bringing) higher security, QoS, packet segmentation and many more IP addresses. (the transition from IP v4 to IP v6 must be supported by the system provider)

ARP protocol
The ARP abbreviation stands for Address Resolution Protocol. This protocol is used to find the physical address (MAC) based on a known IP address. If required ARP sends information concerning the wanted address to all the stations in the network – Broadcast. The stations consequently answer with a message containing their MAC. If the wanted device/station is outside the node/segment, the appropriate router will answer instead of it.

Transport layer
The transport layer is implemented only in terminal devices and it adjusts the behavior of the network according to the requirements of the device/application.

Application layer
The application layer is composed of programs that use net services to fulfill the needs of users. Examples of specific protocols are for instance FTP, DNS and DHCP.
Application protocols use TCP, UDP or both services at the same time. So called ports are used to differentiate between application protocols, they represent a type of label of the application. It is possible to change the ports in the settings of the service, but each service has a default port that isn’t changed for most services and is used as an unwritten standard.

“    FTP = 21
“    DNS = 53
“    DHCP = 67 + 68

Proxy Lists

proxy list is a list of all the working proxy websites. A proxy website is a website which lets you surf anonymously and it allows you to access the websites which have been blocked by a content filter.

How does this work –

A proxy website allows you to enter the URL of the website which you wish to surf and once you’ve entered the URL, the proxy website displays the target website in a special window. In this way, your real IP address, location are not revealed to the website and you can access it even if it is blocked by the network administrator. You can interact with the website in real-time just like the way you would have interacted with it if it was not blocked on your network.

The only known problem associated with the proxy websites is that they’ve a small life as other websites block the proxy websites to avoid anonymous surfing. So, it’s important to maintain a regularly updated proxy list in order to get the information about the working proxy websites.

Proxy.com

A Proxy is a software program that resides in-between a client computer and a server. All communication between the client and the server are routed through the proxy. The proxy software can reside either on a dedicated computer, sandwiched between the client and the server, or exist in the client computer itself. The proxy acts like a filter that sifts all communication that occurs between the client and the server.

Proxies are widely used in the corporate environment to control the internal computer network’s communication with the outside world. The entire data flow is constricted through the proxy for security and policy implementation purposes. Any data that violates company policy or poses a threat to the internal network’s security is immediately filtered. Proxies are also used in educational institutions and public facilities like libraries for the very same purpose.

Proxies can be used for a wide variety of purposes. Based on their functions, they are called by different names. A proxy that is used for monitoring and filtering content, as explained earlier, is known as a Content Filtering Proxy. On the other hand, a proxy that transmits all communication without altering the contents of the traffic is known as a Gateway.

A Caching proxy is one that maintains a cache of frequently requested information. It is basically used to reduce bandwidth usage and to speedup the network. When a client requests a piece of information, the proxy normally passes that request to the server and relays the response from the server back to the client. But if the same piece of information is requested repeatedly, the proxy maintains a copy of that information in its cache and directly sends that information to the client instead of sending a request to the server. This speeds up response time and also reduces the bandwidth requirement of the entire network.