Basics of Internet

Internet is not a network but a set of them. This set of networks provide connection between computers and equipments at a global level. Nowadays mostly all used networks are connected to internet or use the same technology. Why internet is the biggest and the only public network in the world? The simplest answer is all network technologies were designed to be small. Internet was born big and it was designed to be big. Of course some technology advances have been needed but no other network technology has become as big as internet. Once become a standard no other network technology can compete with Internet.

 Ethernet  MAC address  ARP
 Hub  Protocol  DHCP
 Switch  TCP/IP  DNS
 Router  Mask  NAT


It is the most common physical level for data communication. What does physical level mean? It refers to connectors, voltage, speed, etc. Over ethernet it is possible to use any protocol but the most usual one and the one used in Internet is TCP/IP. In these pages other protocols are mentioned but complementary to TCP/IP. An Ethernet network is recognized by connectors. They are similar to telephony ones but a bit bigger. This connector is named RJ45 similar to the telephony one named RJ11. Cables used are classified by category. The needed one is “category 6”, no less than this. Maximum speed is 100 MBit/S or even 1000 MBit/S ( 1 GBit/S ). At professional level there are 10 GBit/S networks but connection is done with fiber optics.


It is an obsolete element but mostly used in ethernet networks in the past. All computers are connected to the hub allowing connection between all computers. All computers are connected with all. Every time one computer wants to transmit it waits until no one is transmitting. In this moment it starts transmitting and all the rest are listening to. As the transmission has a destination address only the destination computer receives the connection. The other ones ignore the message. It is possible two computers start transmitting at the same time. This situation is detected and both computers stop transmitting and try a bit later. Two hubs can be connected between them allowing more computers connection. Anyway this element is not used and switches are used instead as they are more efficient.

MAC address

It is the address used in ethernet networks. Every equipment, computer or device have a different MAC (Media Access Control) selected in the factory and impossible to change. If one computer have several ethernet adapters each one has a different MAC address. All ethernet messages have origin MAC and destination MAC. There is only an exception: “broadcast” message. This broadcast message has origin MAC and it is sent to all computers in the network.


It is hub evolution working in a more efficient way. Hub allows only one connection at the same time in all computers connected. Switch allows several connections at the same time using MAC address to send the messages to the right computer without sending to the rest. When a computer send a message the switch check the origin MAC address maintaining a table of the MAC addresses in each connector. Switch only send the message to the connector where the destination MAC address have been detected. As the message is not sent to the rest of computers they think the network is idle and start new connections. Switch take care every message goes to the right computer controlling all possible communications at the same time. Only broadcast messages are sent to all computers but they use to be few ones. Same as hub several switches could be connected increasing the size of the network but this connection should be done carefully in order to avoid loops.


A protocol describe the procedures for sending messages through a physical level. Consider the classical mail. Physical level would be envelops, postmans, trucks, etc. Protocol indicates how we should insert the message in the envelope, how to write the address, the right stamps, origin address in the other side of the envelope, etc. It is possible a protocol inside other one. Imagine you send a mail to a big company and, once the mail received in the company, it should be delivered internally through internal mail (with a different protocol).

In Internet world the most used protocol is named TCP/IP but it is not the only one. TCP/IP is the base protocol but Web connections use HTTP protocol working over TCP/IP. For protocol analysis there is a really powerful and free application called  Wireshark We can use this application to analyze and understand how protocols are used in our computer communication.


TCP/IP is the most used protocol nowadays. Explaining TCP/IP in some lines is so ambitious but, as a glance, it is based on all computers and equipments have an address called IP address. Every IP address is a set of four numbers from 0 to 255. One example of IP address is Every message have the origin IP address and destination IP address. The network is in charge of assuring any message reach destination address.


It is the protocol used before TCP/IP communication. When a computer wants to send a message to a specific IP address first it is needed to know its MAC address because this is the only valid one in Ethernet. Using ARP protocol sending computer send a broadcast message to all computers asking for which computer has the specific IP address. Only the right computer answers indicating its MAC address. Computers maintain a table with the relationship between IP address and MAC address and ARP protocol is used only the first time or when something wrong.


Routers are network elements connecting two or more different networks even with different technologies. It works at TCP/IP level transmitting messages from one network to another one. Routers don’t analyze a message to check if it should be sent to another network. Every computer send directly the message if the destination computer is in the same network. If destination computer is in another network sending computer sends the message to the router. Router has an IP address in every network connected to and all computers know the IP address of router (normally it is named gateway IP address in configuration). Routers discover how networks are connected even communicating with other routers allowing proper routing of the messages.


It is the key information computer needs in order to decide if destination address is in the same network or not. Usual mask is “”. Any other mask is used by expert people. As we have explained IP address is a set of four numbers from 0 to 255. This mask means the first three indicates the network and the last one indicates computer. If our computer has the IP address and the destination address is both are in the same network as three first numbers are equal. If destination address is both computers are in different networks and message is sent to the router. Usual address for the router would be


DHCP protocol is complementary to TCP/IP and it is used when a computer is connected or switched on. It is used to automatically configure TCP/IP in the computer. Normally our home router is a DHCP server as well. If we configure our computer to be configured automatically, when switched on, it looks for a DHCP server. DHCP server send back TCP/IP configuration for this specific computer. This configuration is the IP address, mask, router IP address and DNS IP addresses. DHCP server provide different IP addresses for each computer maintaining a list of all connected computers and assigned IP address.


IP addresses are difficult to use. It would be really difficult to remember the web page of a newspaper in case it is But it is easier if we have to write The system translating is called DNS (Domain Name Server). When we write our computer connect automatically with configured DNS server and it send back the IP address of the newspaper needed to communicate. All internet providers have their own DNS server (It use to be two) but any DNS could be configured.


NAT stands for “Network Address Translator”. NAT is a functionality use widely normally in routers. When we connect our computer to our home router the assigned IP address use to be 192.168.X.X. All routers assign IP addresses in the same range. This network in our home our private network and these IP addresses cannot be used in Internet as al private networks use the same IP addresses. But our router is connected to Internet with an IP address (only one) called public IP address. This public IP address is unique in Internet. When a computer send a message to internet it sends it to the router and the router send it to Internet. But origin IP address is a private one not valid in Internet. Router change in the message the private IP address by router public IP address. With this change router assure it will receive answer messages. When receive the answer router change destination IP address by the right private one and send to the right computer. All these changes are made by NAT functionality. As several computers connect to Internet simultaneously router maintain a table in order to distinguish in any answer message what is the right destination computer. When router receives a message not corresponding to any sent message this message is simply dropped. This is a fantastic security functionality because any attack is dropped as it doesn’t correspond to any outgoing connection. Other main advantage of NAT is it is possible to connect to Internet a big number of computers using only a single public IP address. IP addresses for private networks are reserved to the following ranges: 10.X.X.X, 172.16.X.X to 172.31.X.X, 169.254.X.X and 192.168.X.X. Any address in these ranges is a private address and it is not used in Internet. These ranges are big enough for any private network size.