CCENT (ICND1)MaintenanceManagementNetworking 101

Useable IP addresses in private networks

I’m sure you can imagine that trying to administer an IP address for every single person on the internet would be near impossible. Added to the fact that companies don’t want every computer in their network to be accessible on the internet for security reasons. The Internic and IANA (the two governing bodies for IP addresses and Port numbers), had to come up with a way for everyone to be able to choose their own IP address for private use. The IP addresses we are going to be discussing here fall part of RFC 1918 (The Request For Comment that resulted in the industry standard).


Class A –


Class B –


Class C –


The IP addresses above are allowed to be used inside your private networks, whether this for home use or company use, may NOT appear on the internet. Since everyone on the planet uses the above guidelines, you can imagine that more than one person has the IP address, and if this was to ‘appear’ on the internet, there would be multiple conflicts.


For example, at my house I am using the IP address range of – for all my computers and networking devices. However in order for me to surf the web from my house, something has to translate my internal IP addresses, to a single Public IP address (that my Internet Service Provider gives me). This process is known as NAT (Network Address Translation) and is usually performed by a Router, in my case it’s my ADSL router that connects to my phone line. The other major advantage of running this system, is that my house only uses one public IP address, even though I may have hundreds or thousands of devices that use internal IP addresses. If you think about this from a company perspective, which might have tens of thousands of devices with internal IP addresses, they only need one IP address for internet access, and hence the ISP’s only have to issue and administer a single IP address for a large organisation.

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