IP addresses: IPv4, IPv6, public and private

Every device that talks to the internet needs a number, not a name. The core specification of the Internet Protocol, RFC 791, says addresses “distinguish sources and destinations to the host level”, meaning each device gets its own number so packets know exactly where to go. Names like example.org are a convenience for humans, resolved to a number by DNS (covered in its own course); the network itself only understands numbers.

IPv4: four billion addresses, and not enough

The original format, IPv4, uses a 32-bit address, written as four numbers separated by dots, such as 192.0.2.1. RFC 791 defines it as “fixed length of four octets (32 bits)”. Thirty-two bits allows for about 4.3 billion unique combinations, as Cloudflare’s Learning Center explains. That sounded huge in the 1980s. It is not enough for a world of billions of phones, laptops, routers and smart devices, which is why a newer, much larger format exists.

IPv6: room for everyone, for a long time

IPv6 addresses are 128 bits long, written as groups of hexadecimal digits separated by colons, such as 2001:0db8:85a3:0000:0000:8a2e:0370:7334. As RFC 8200 puts it, IPv6 “increases the IP address size from 32 bits to 128 bits, to support more levels of addressing hierarchy, a much greater number of addressable nodes, and simpler autoconfiguration.” Both versions run side by side today: your Mac usually has one address of each kind.

Public and private: the address only your router knows

Not every address needs to be reachable from the whole internet. RFC 1918 sets aside three ranges of IPv4 addresses for use inside homes, offices and other private networks:

• 10.0.0.0 to 10.255.255.255
• 172.16.0.0 to 172.31.255.255
• 192.168.0.0 to 192.168.255.255 (the range most home routers use)

RFC 1918 is explicit that these addresses stay local: “because private addresses have no global meaning, routing information about private networks shall not be propagated on inter-enterprise links, and packets with private source or destination addresses should not be forwarded across such links.” Internet providers are expected to reject any attempt to route them onto the public internet. Only one address, usually held by your router, needs to be a public one that the rest of the internet can find; how that sharing works is the subject of the next lesson.

$ ipconfig getifaddr en0
192.168.1.12

192.168.1.12 falls inside the private range above: it identifies this Mac on its own Wi-Fi network, but no computer on the wider internet can send a packet directly to it. Cloudflare’s Learning Center notes that most home connections also get a temporary, “dynamic” public address from the ISP rather than a fixed one, chosen from a shared pool because permanent addresses are more expensive to maintain for both the provider and every customer.