Nonroutable IP Addresses

If you do not know what an IP address is, turn back now, and learn about those first, because none of this will make sense otherwise.

Ok, those of you still with me, good for you. We're going to breifly review IP Addresses' now.

Every interface on the internet must have an IP address assigned to it, according to the TCP/IP Protocol. Most of these, are routable IP adresses, meaning that they have an IP that can be routed (Duh). What does "Routable IP Address" Mean? Well, only routable IP addresses are allowed on the Internet. Meaning that you can directly address them, through their home router

However, nonroutable IP addresses cannot exist on the internet, because they are not allowed to. Why have nonroutable IP addresses, if they cannot exist on the internet? Because they are highly useful for intranets.

Suppose your company has 10 computers, but only one IP given to you by your ISP, yet you need to use all computers online at the same time? According to the TCP/IP protocol, only one machine at a time may occupy a IP on a single subnet. So, you have one machine accept the routable IP, and then use NAT to allow the other 9 computers to use nonroutable IP addresses. This allows all computers to be routed through the same IP, which is then truly routed to the Internet.

I told you it was confusing.

Now, you might be asking what IP's are nonroutable. By definition, any IP that falls within the range of 10.0.0.0 and 10.255.255.255; 172.16.0.0 and 172.16.255.255; 192.168.0.0 and 192.168.255.255 (inclusive), is nonroutable. Another nonroutable IP range is 127.0.0.0 to 127.255.255.255, because that is the loopback IP range

More to come on this as I can think what I forgot.

According to RFC 1918: "Address allocation for private internets", these fall within "Reserved Address space". In other words, IANA doesn't care about who uses these.

Further the RFC states, in part:

Indirect references to such addresses should be contained within the enterprise. Prominent examples of such references are DNS Resource Records and other information referring to internal private addresses. In particular, internet service providers should take measures to prevent such leakage.

• 10/8 (10.0.0.0 - 10.255.255.255)
• 172.16/12 (172.16.0.0 - 172.31.255.255)
• 192.168.0/16 (192.168.0.0 - 192.168.255.255)

Note that an IP address is required for each interface not just a computer. Any routable interface must have an IP address. There's a subtle difference.