IPv4 calculations

Revision as of 22:30, 15 January 2014 by 404 Error (Talk)

WARNING: Work in progress. Do not edit this article unless you are the original author.


Refresh on TCP/IP model

When the ARPANet (a packet oriented network) was born in those good old seventies, engineers had to solve the problem of making computers being able to exchange packets of information over the network and they invented in 1974 something you are now using to view this page: TCP/IP! TCP/IP is a collection of various network protocols, being organized as a stack. Just like your boss does not do everything in the company and delegates at lower levels which in turn delegates at an even more lower level, no protocol in the TCP/IP suite takes all responsibilities, they are working together in a hierarchical and cooperative manner. A level of the TCP/IP stack knows what its immediate lower subordinate can do for it and whatever it will do will be done the right way and will not worry about the manner the job will be done. Also the only problem for a given level of the stack is to fulfill its own duties and deliver the service requested by the upper layer, it does not have to worry about the ultimate goal of what upper levels do.

<illustration goes here TCP/IP model>

The above illustration sounds horribly familiar : yes, it is sounds like this good old OSI model. Indeed it is a tailored view of the original OSI model, and it works the exact same way.

The postal mail service

If you are not super familiar with OSI or TCP/IP models let's take an analogy with the national postal service because it works in a similar way. Suppose you want to send a parcel to your cousin located in another in a town in the same country (to keep things simple the parcel won't go through customs or firewall if your prefer) via the national postal service. So basically you (the application), take a box and pack what you want to send then you write the address of the destinee on the box, then give the box to the clerk at your post office outlet. The clerk will asks you for the level of service you want (standard or registered mail) then we will put your parcel on a rack where where other parcels sent by some other people lies. All of those parcels will then be transported (routed) from the post office outlet to the nearest mail processing facility where they is transported (routed) to another postal facility and so on using different paths over the country until each one reaches its destinee's postal outlet. From your point of view, your only matter it to see your parcel delivered to the right destinee. You know the postal service will do its job, you don't have to worry about how it will do it.

For a computer this is more or less the same:

  • You : Application
  • The box contents : The data to be transferred by the application
  • Transport reliability level : Transport (Standard => UDP, Registered => TCP)

Classful and classless networks

Who would ever need millions of addresses afterall?  So in theory with those 32 bits we can have around 4 billions of computers within that network and arbitrarily retain that the very first connected computer must be given the number "0", the second one "1", the third one "2" and so on until we exhaust the address pool at number 4294967295 giving no more than 4294967296 (2^32) computers on that network because no number can be a duplicate. 


Those addresses follows the thereafter logic:

32 bits (fixed length)
Network part (variable length of N bits ) Host part (length : 32 - N bits)
  • The network address : this part is uniquely assigned amongst all of the organizations in the world (i.e. No one in the world can hold the same network part)
  • The host address : unique within a given network part

So in theory we can have something like this (remember the network nature is not to be unique, it hs to be be a collection of networks  :

  • Network 1 Host 1


Just like your birthday cake is divided in more or less smaller parts depending on how guests' appetite, the IPv4 address space has also been divided into more or less smaller parts just because organizations needs more or less computers on their networks. How to make this possible? Simply by dedicating a variable number of bits to the network part! Do you see the consequence? An IPv4 address being always 32 bits wide, the more bits you dedicate to the network part the lesser you have for the host part and vice-versa, this is a tradeoff, always. Basically, having more bits in :

  • the network part : means more networks possible at the cost of having less hosts per network
  • the host part : means less networks but more hosts per network

It might sounds a bit abstract but let's take an example : imagine we dedicate only 8 bits for the network part and the remaining 24 for the hosts part. What happens? First if we only


Is the network part assigned by each organization to itself? Of course not! Assignment are coordinated at the worldwide level by what we call Regional Internet Registries or RIRs which, in turn, can delegate assignments to third-parties located within their geographic jurisdiction. Those latter are called Local Internet Registries or LIRs (the system is detailed in RFC 7020). All of those RIRs are themselves put under the responsibility of now now well-known Internet Assigned Numbers Authority or IANA. As of 2014 five RIR exists :

  • ARIN (American Registry for Internet Numbers) : covers North America
  • LACNIC (Latin America and Caribbean Network Information Centre): covers South America and the Caribbean
  • RIPE-NCC (Réseaux IP Européens / or RIPE Network Coordination Centre): covers Europe, Russia and middle east
  • Afrinic (Africa Network Information Center) : covers the whole Africa
  • APNIC (Asian and Pacific Network Information Centre) : covers oceania and far east.