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8 The Internet Protocol version 6

This is the new big thing! Since there are so few addresses representable in 32 bits (only 4,294,967,296 not counting the reserved ones), the Powers That Be decided we needed a new addressing scheme. One with more bits. One that could last, for all intents and purposes, forever.

There was a problem: we were running out of IP addresses. Back in the 1970s, a world with billions of computers was beyond imagination. But today, we’ve already exceeded this by orders of magnitude.

So they decided to increase the size of IP addresses from 32 bits to 128 bits, which gives us 79,228,162,514,264,337,593,543,950,336 times as much address space. This should genuinely last a looooooong time.

Lots of this address space is reserved, so there aren’t really that many addresses. But there are still a LOT, both imperial and metric.

That’s the main difference between IPv4 and IPv6.

For demonstration purposes, we’ll stick with IPv4 because it’s still common and a little easier to write out. But this is good background information to know, since someday IPv6 will be the only game in town.

Someday.

8.1 Representation

With that much address space, dots-and-decimal numbers won’t cut it. So they came up with a new way of displaying IPv6 addresses: colons-and-hex numbers. And each hex number is 16 bits (4 hex digits), so we need 8 of those numbers to get us to 128 bits.

For example:

2001:0db8:6ffa:8939:163b:4cab:98bf:070a

Slash notation is used for subnetting just like IPv4. Here’s an example with 64 bits for network (as specified with /64) and 64 bits for host (since 128-64=64):

2001:0db8:6ffa:8939:163b:4cab:98bf:070a/64

64 bits for host! That means this subnet can have 18,446,744,073,709,551,616 hosts!

There’s a lot of space in an IPv6 address!

When we’re talking about standard IPv6 addresses for particular hosts, /64 is the strongly-suggested rule for how big your subnet is. Some protocols rely on it.

But when we’re just talking about subnets, you might see smaller numbers there representing larger address spaces. But the expectation is that eventually that space will be partitioned down into /64 subnets for use by individual hosts.

Now, writing all those hex numbers can be unwieldy, especially if there are large runs of zeros in them. So there are a couple shortcut rules.

  1. Leading zeros on any 16-bit number can be removed.
  2. Runs of multiple zeros after rule 1 has been applied can be replaced with two sequential colons.

For example, we might have the address:

2001:0db8:6ffa:0000:0000:00ab:98bf:070a

And we apply the first rule and get rid of leading zeros:

2001:db8:6ffa:0:0:ab:98bf:70a

And we see we have a run of two 0s in the middle, and we can replace that with two colons:

2001:db8:6ffa::ab:98bf:70a

In this way we can get a more compact representation.

[This is “good to know” information, but just file it away under “IPv6 automatically gives all interfaces an IP address”.]

There are addresses in IPv6 and IPv4 that are reserved for hosts on this particular LAN. These aren’t commonly used in IPv4, but they’re required in IPv6. The addresses are all on subnet fe80::/10.

Expanded out, this is:

fe80:0000:0000:0000:0000:0000:0000:0000

The first 10 bits being the network portion. In an IPv6 link-local address, the next 54 bits are reserved (0) and then there are 64 bits remaining to identify the host.

When an IPv6 interface is brought up, it automatically computes its link-local address based on its Ethernet address and other things.

Link-local addresses are unique on the LAN, but might not be globally-unique. Routers do not forward any link-local packets out of the LAN to prevent issues with duplicate IPs.

An interface might get a different IP later if a DHCP server hands one out, for example, in which case it’ll have two IPs.

8.3 Special IPv6 Addresses and Subnets

Like with IPv4, there are a lot of addresses that have special meaning.

There are IPv6 ranges with special meanings, but those are the common ones you’ll see.

8.4 IPv6 and DNS

DNS maps human-readable names to IPv6 addresses, as well. You can look them up with dig by telling it to look for an AAAA record (which is what DNS calls IPv6 address records).

$ dig example.com AAAA
; <<>> DiG 9.10.6 <<>> example.com AAAA
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 13491
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;example.com.            IN    AAAA

;; ANSWER SECTION:
example.com.        81016    IN    AAAA    2606:2800:220:1:248:1893:25c8:1946

;; Query time: 14 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Wed Sep 28 16:05:16 PDT 2022
;; MSG SIZE  rcvd: 68

You can see the IPv6 address of example.com in the ANSWER SECTION, above.

8.5 IPv6 and URLs

Since a URL uses the : character to delimit a port number, that meaning collides with the : characters used in an IPv6 address.

If you run your server on port 33490, you can connect to it in your web browser by putting the IPv6 address in square brackets. For example, to connect to localhost on address ::1, you can:

http://[::1]:33490/

8.6 Reflect

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