IPv4 Addresses & Private Ranges

An IPv4 address is 32 bits long, written as four octets — four groups of 8 bits — separated by dots. Eight bits can express 256 values, so each octet runs from 0 to 255. That is why 192.168.10.45 is a valid address and 192.168.10.300 is not.

The address on its own is only half the story. Lesson 2 introduced the subnet mask as the thing that tells a device which leading part of an address identifies the network and which trailing part identifies the individual device on it. The mask is written either in full, as 255.255.255.0, or as a CIDR prefix — a slash and the number of network bits, as in /24. A /24 means the first 24 bits name the network and the remaining 8 name the host within it.

Most IPv4 addresses are public: globally unique, allocated by a registry, and routable across the Internet. But three blocks are set aside by a standard called RFC 1918 for private use. Any organisation may use them on its internal network without asking anyone, and no router on the public Internet will carry them.

That is a deliberate trade. Because everybody reuses the same private space, the world does not run out of addresses nearly as fast — but a device holding a private address cannot be reached from the Internet directly. It gets there through NAT, Network Address Translation, performed by the router at the network edge, which swaps the private source address for a public one on the way out and reverses the swap on the way back. Why it is needed is the point here; how it works is a later lesson in IP Services.

The loopback range is the device talking to itself; 127.0.0.1 in particular never leaves the machine, which makes it useful for testing whether the local networking stack works at all. The link-local range, also called APIPA, is what a host assigns itself when it asks for an address by DHCP and nothing answers. That makes it a diagnostic in its own right: seeing a 169.254 address on a machine is a strong signal that DHCP failed, not that someone configured it oddly.

• An IPv4 address is 32 bits in four octets, each 0 to 255.
• The mask, written in full or as a CIDR prefix, splits the address into a network portion and a host portion.
• The private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 — reused everywhere, routed nowhere on the public Internet.
• NAT at the network edge is what lets a privately addressed device reach the Internet.
• 127.0.0.0/8 is loopback. A 169.254 address means DHCP did not answer.

This is the on-ramp to Week 2, which opens by turning the mask from an idea into arithmetic you can do on paper.