Abstract.

Between all the parameters for the formulation of the access to the net, the netmask it is between the elements mainly encircled from a aura mystery and that, of usual, it comes left 255.255.255.0 without not to know why. This document tries to explain with a comprehensible language, what the netmask is and in which way computer uses such information for sending data over the net.

The ill-famed one triade: IP, gateway and netmask.

Nearly all are capitato, at least once, of imbattersi in the cosi said IP address that is in that one quaterna of numbers, comprised between 0 and 255, separate to you from a point. Even made curious then from this object a po' has been searched on on the net meant its until discovering that the IP others is not if not the address univoco of every public machine on Internet.

IP address is therefore, in the virtual world of Internet, what the address mails them is in the life every day. In the real world the address mails univocamente identifies them an adressee (and sender) of a way letter that this can reach the only real adressee between all the billions of possible adressees. In the virtual world IP address univocamente identifies the sender and the adressee of a particular called object package TCP/IP.

Therefore as a letter contains, on the envelope, the address mails them of the sender and of the adressee, in the world of the TCP/IP, every package it contains on the envelope, call in this case "header", IP address of the sender and the adressee.

The information that the two computers want to be exchanged and that, nel real world, would make the veci del contained della letter, is called, nel world goddesses computer "payload".

Therefore two computers in order to communicate exchange packages TCP/IP containing in the header their respect IP addresses to you and in payload the true and own information.

So that all these functions without ambiguity are necessary that the allocation of the addresses publics of Internet is univoca and is not therefore possible gives an IP address arbitrarily. In fact, like in the real life, also in that virtual one we cannot invent an address but it comes assigned to us.

Mask of configuration TCP/IP on windows

Once in possession of IP address to assign to our calculating we can open the relative shielded one to the configuration of the net. But any is our operating system will always find of forehead at least three parameters: already cited IP address, netmask and the gateway of default and until the configuration of these parameters it is not correct and coherent we will not be in degrees to approach the net resources.

But if in the real life, the address mails them enough for being able to send and to receive a letter, why on Internet IP address is not sufficient?

As the computer to the shipment of a package reasons on the net.

In order to understand why single IP address is not a sufficient information for being able to interact with other computers of the net we must follow the logical process that door to the creation of a package. We consider therefore that our S computer (source) with IP address, as an example, 195.32.69.2 must send to a information to a D computer (adressee) that for hour we identify with a generic IP address AAA.BBB.CCC.DDD.

Like saying, the information becomes therefore "payload" of a package TCP/IP and comes therefore inserted in one particular envelope TCP/IP. On the envelope it comes put the address of the sender, 195.32.69.2 and that one of the adressee.

We imagine, to this point, the calculating with the envelope in hand that os in the cassette mails them more neighbor. That that we would see, to this point, and that it finds itself to having to make one chosen.

In the real case the cassette has two fissures, one for the destined mail to the own city and the other for all the other destinations. The same thing happens in the net. The computer must decide if the letter is destined to a calculating of the own local net or if the adressee is "more far away". Moreover, while in the real case the choice can also be committed an error and be hoped that the letter comes equally recapitata, in the computer science case must be exact.

But why it must make this choice? Why the package goes dealt in different way to second of the various destination. In fact if the package is destined to the local net it can already be thrown cosi com' is on the net in how much the D computer, ivi in listens, will see the package to R-in.transito destined he and it will recover it. In the case in which the adressee it is remote, the package cannot be thrown cosi com' is on the net in how much D is not in listens on it. The package, in this last case, goes modified for being given in meal to a particular calculating of the local net, the gateway of default. And here therefore that ago its appearance the second fundamental parameter of the configuration of the net.

The gateway identifies the access to the external world and is to knowing like catching up the rest of Internet and is therefore to it that we must send the package so that comes correctly forwarded to D. Allora without one corrected configuration of the gateway of default could even catch up correctly our local computers but the rest of the world would be interdetto.

Once cleared therefore why of the necessity of the choice, it must clear like. How ago our PC to understand if the adressee is local or is remote?  In the real world the deductive process would be simple:

S lives in the city X. D lives in city X?

If the answer to this question is positive then the letter it goes sended in the fissure for the same city otherwise goes imbucata in the other. In the virtual case the question is similar:

S is found in the net X. D is found in net X?

The positive result or negative of this question decides the way with which the package it goes sended, if to a local computer or the gateway of default.

The netmask it resolves the problem of what is local and what is remote.

The question to which we must therefore know to answer still contains but one incognito. S has IP address 195.32.69.2 but what agrees for net X?

In order to calculate net X not enough the acquaintance of single IP address. Even if more scaltri they could answer that the net to which IP address 195.32.69.2 belongs is net 195.32.69.0 and this answer is often that right one, in the world of the computers enough not to often guess to us, must always guess to us.

You will sure have understood that in order to gain the net address to which an IP address it belongs there is need to know the netmask. The netmask, identifying the net to which an IP address it belongs, it identifies what is local and what is remote. Moreover the identification of what is local from also a visualization of qual' is the interval of local IP addresses and therefore of how much it is or it can be large our net in the sense of the number of computers that it can contain.

A little binary mathematics.

Hour we do not have to be scared but in order to understand in that way from the acquaintance of IP address and netmask the drifts the address of the net we must penetrate a po' in a world formed from zeri and uni, the world of the binary mathematics.

We begin to know this world making the acquaintance with the two main actors, Mr. ONE and Mr. ZERO. Thanks to these two symbols are possible to define whichever number and what we get ready to ourselves to make hour is to see like this thing, cosi from the first moment strange, it can work.

In the numeration system that we all know ten symbols enter in game with which we write all the numbers. These ten symbols are:

that it arranges with with one logical said posizionale to you they allow us to write a whichever number. For what it is "the posizionale" adjective? We try to understand it with an example. When we want to write the year of birth of following undersigned we the put one of flank to the other symbols

Our perception of this sequence of figures is a posizionale perception nel sense that this way to write from a different weight alle single been involved figures cosi that number 9 in second position from left does not have the same value dell' other 9.

Every position to the left of an other "hung" ten times more than that one to right and therefore taken the numbers over from right will be had that that number indicates nine times more once one more six times ten more nine times hundreds thousands. E' this and only this that we indicate when we write "1969".

Ten symbols, posizionale increment of powers of ten, all this to repeat itself of value ten from the name to our called numerical system "decimates them".

Usually one says that the fact to have a system decimates them drift from the fact that we have ten fingers and therefore I use it of ten symbols it is only a convention and it is in truth possible to use an arbitrary number of symbols in order to write the numbers. In truth, the cosi said formed binary system exactly from the single symbols

the calculating covers a fundamental importance in the world of the computers in how much, working with currents or lives magnetic is alone in a position to less finding the presence or the absence of one of these fields in a memory element and the survey or than such fields it comes associated to the reading in the cell of memory of one or one zero.

But inasmuch as our brain is accustomed to the system decimates them, we see in the first place as a correspondence between the first numbers of the system is constructed decimates them and of that railroad.

Zero and one is obviously equal in all and the two systems. With the two we have the first difficulty. While in the system it decimates them exists an only symbol that identifies such value univocamente, in the binary system we have already exausted all the esprimibili numbers with a single figure and therefore we must pass to two figures. As in the case it decimates them in which after the nine it comes the ten, also in the binary case, after the 1, it is passed having to use two figures and therefore the binary corrispettivo of the two decimates them e' the 10. The three railroad decimates them will be the 11 and with the four we have ended also all the ways to express in railroad a number with two figures and therefore we pass to three and therefore the the 100 four decimates them e' railroad.

Reassuming and extending these associations, indicating with the pedice the type of relative numeration, will be had that

0₁₀ = > 0_2, 1₁₀ = > 1_2,

2₁₀ = > 10_2, 3₁₀ = > 11_2,

4₁₀ = > 100_2, 5₁₀ = > 101_2, 6₁₀ = > 110_2, 7₁₀ = > 111_2,

8₁₀ = > 1000_2, 9₁₀ = > 1001_2, 10₁₀ = > 1010_2, 11₁₀ = > 1011_2,

12₁₀ = > 1100_2, 13₁₀ = > 1101_2, 14₁₀ = > 1110_2, 15₁₀ = > 1111_2,

Obviously also the binary system is posizionale and, as in the numeration it decimates them a position on the left is ten times greater of that one to right, analogous in the binary system, a position on the left is two times greater of that one to right for which, as an example, 100111₂ means, leaving from right, once one more once two more once four more once trentadue and therefore 39₁₀.

Well, hour that we know the point of view of the calculating R-with regard to the numbers, we are in a position to watching IP address from a new point of view putting to us in the cloth of a computer.

When we human beings say that a calculating has address 195.32.69.2 which the computer sees is four sequences of eight binary symbols separates to you from a point. Every binary symbol 0.1 comes called in jargon "bit" and eight bit goes to form a new called element "byte". Therefore IP address can be seen like a sequence of 4 byte or, that it is the same thing, of 32 bit.

Why we have said that every element of the sequence is formed just from 8 bit? Why we have already said that the numbers separate you from the point can go from 0 to 255 and 255 in railroad are

That this is just the value that represents 255 in railroad sees without having to add all the terms over with a simple one trucchetto. Taken the number

that it is the successive one of the number over has that this is 2⁸ = 256 in decimates them and therefore the number over is 2⁸-1 that is just 255.

To this point, for pure delight we show our IP address used in the example from the point of view of the binary system