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Course: IP Version 4 Addressing And Subnetting Deep Dive (Part 4)


5 months ago4 min read

Hello and welcome, I am currently working on a video course 'addressing' IP version 4, and I've decided to post my slides and scripts here on STEEM as I go along. I appreciate any feedback or suggestions, or if you are new to the world of computer networking, I hope you enjoy them! Here is part 4 of a multi-part series.

If you missed any sections, click one of the links below for your favorite front end:


When connecting to the public Internet, you will be assigned a public IP address or range of addresses. These addresses will be both visible and routable on the public Internet.

In some cases you might want to create a network isolated from the public Internet. For this purpose, you can allocate a private IP address or range. These IP addresses ARE NOT visible or routable on the public Internet. Which IP addresses are considered private are defined by RFC. More on that later.


If you require a public IP address or range of addresses, the primary method is to obtain an allocation from your Internet Service Provider (ISP). The ISP itself receives a large block from a Regional Internet Registries (RIR), they can in turn allocate to their customers. The ISP owns these IPs so you cannot take them with you if you decide to move to a different service.

If you are an enterprise that must connect to multiple Internet service providers, also called being multi-homed, you must request a direct allocation from an RIR. This is because advertising the IP addresses owned by one ISP to another ISP can cause routing problems. Allocations made by RIRs are considered portable, meaning you can change ISPs without having to change your IP addresses.


Here is a map of the Regional Internet Registries across the globe. We have:

  • AfriNIC serving the African continent
  • APNIC serving the Asia-Pacific region
  • ARIN serving the US and Canada
  • LACNIC serving Latin America
  • RIP NCC serving Europe and the Middle East

img src
Rir.gif: Dork BlankMap-World6,_compact.svg: Canuckguy et al. derivative work: Sémhur


Private IP address ranges can be allocated by anyone to any network. They are non-routable over the public Internet, and it is a standard practice to block them. Because the IPs are private, duplicate or overlapping addresses can be allocated by different networks. If you have a private network address and require access to the Internet, some form of network translation or proxy must be used. Private addressing is defined by two RFCs, 1918, and 3297 for which will will go into more detail.


Private IP addresses fall into a category of IP addresses referred to as special-purpose ranges. These include the RFC 1918 address space, used for private networks, the local host range used for network testing, and the APIPA address space, used for automatic and dynamic IP addressing of interfaces.


RFC 1918 private addressing became a published standard in 1996. It was in part created to address the problem of public IP address exhaustion, and also to provide a range of private IP addresses that wouldn’t interfere with the public Internet.

For this purpose, a full class A, 16 class B, and 256 class C networks were dedicated for private addressing.

The included table lists each class, the available networks for that class, along with the CIDR notation for the available networks. A network administrator is free to choose any of these networks, and divide them up using variable length subnet masks.


Another for of private addressing is defined in RFC 3297, also referred to as Automatic Private IP Addressing (APIPA) . This method creates what is referred to as link-local addresses. This means an interface will dynamically and automatically assign an IP address using an IP address from the APIPA range of 169.254/16. APIPA can be used to create ad-hoc networks by simply plugging unconfigured devices into the local area network.

This feature is implemented in many operating systems, such as Microsoft Windows, Mac OS, and some versions of Linux by default. If you’ve ever experienced network connectivity problems, you may have seen one of these addresses.


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