IPv4 vs IPv6: public, private and shared addresses

Two address sizes

An IP address is the number that tells the internet where to deliver a packet. The original Internet Protocol, IPv4, defined in RFC 791, fixes every address at four octets, which is 32 bits. That allows 2^32 different values: 4,294,967,296 in total, which sounds like a lot until you count every phone, laptop, server and router that wants one.

IPv6, specified in RFC 8200, raises the address size from 32 bits to 128 bits. The standard gives three reasons: more levels of addressing hierarchy, far more addressable nodes, and simpler automatic configuration. 2^128 is about 3.4 × 10^38 addresses, so IPv6 networks are not rationed the way IPv4 is.

How they are written

IPv4 is written as four decimal numbers from 0 to 255, separated by dots, such as 203.0.113.45. Each number is one octet, or 8 bits.

IPv6 is written as eight groups of four hexadecimal digits separated by colons. Two shortcuts keep it readable. Leading zeros in a group can be dropped, and one run of all-zero groups can be replaced by a double colon. RFC 4291 uses this example: 2001:DB8:0:0:8:800:200C:417A can be shortened to 2001:DB8::8:800:200C:417A. The loopback address, which always means this computer, is ::1.

RFC 4291 also fixes the shape of most IPv6 addresses: for ordinary unicast addresses the last 64 bits, called the interface identifier, name the device, and the first 64 bits name the network. That is why a home IPv6 network is usually a /64.

Private addresses and the router in between

RFC 1918 sets aside three IPv4 blocks for private networks. Anyone may use them inside a home or company, and they are never assigned to anyone on the public internet:

• 10.0.0.0 – 10.255.255.255 (10/8): 16,777,216 addresses
• 172.16.0.0 – 172.31.255.255 (172.16/12): 1,048,576 addresses
• 192.168.0.0 – 192.168.255.255 (192.168/16): 65,536 addresses

Worked example: one home, three addresses

Picture a laptop on a home network. The router hands it the private address 192.168.1.23. The ISP gives the router one public IPv4 address, here written as the documentation address 203.0.113.45. Every website the laptop visits over IPv4 sees 203.0.113.45, not 192.168.1.23, and so does every other device in the house.

If the ISP also offers IPv6, the router receives a network prefix such as 2001:db8:4a1:2c00::/64 and the laptop builds its own address inside it, for example 2001:db8:4a1:2c00:1c3e:9f2:aa01:7d. That address is public: no translation is needed, because IPv6 has enough room for every device. A website reached over IPv6 sees this address instead of the IPv4 one, and the browser picks which family to use for each site.

The shared range used by ISPs

Some ISPs no longer have enough IPv4 addresses to give every customer one. They put a second translator, called carrier-grade NAT (CGN), inside their own network. RFC 6598 reserves 100.64.0.0/10, about 4.2 million addresses, for the link between that CGN and customers' routers.

This shared address space is not globally routable, like RFC 1918 space, but it is meant only for service-provider networks. If your router's WAN or internet address starts with 100.64 to 100.127, your public IPv4 address belongs to the ISP's translator and is shared with other customers. That matters for anyone who wants to run a server at home, which is the subject of the port forwarding lesson.