How To Subnet IPv6

IPv6 was designed to replace IPv4. The simple reason is that the IPv4 address space is running out. The world has reached the point where there are not enough 32-bit addresses to link every device which wants to connect to the Internet. IPv6 uses 128 bits. So how large is the IPv6 address space? Intuitively it might seem like it’s 4 times bigger (128 compared to 32 bits), but it’s actually larger than that. Much much larger. Each additional bit adds an additional ‘power of 2’ to the address space. IPv6 allows almost 8*1028 times as many addresses as IPv4. Basically, a number too large for the human mind to grasp.

Because of the enormity of the address space, IPv6 subnetting design follows different best practice. Most experienced network engineers have become conditioned to using CIDR, VLSM, NAT and private addresses to conserve IPv4 space. With IPv6, conserving addresses is no longer a concern. The subnet that is recommended for end host use in IPv6 is always a /64. This means that even if you have 200 devices on a single /64, you still have 264-200 of unused space. This can be a little hard to accept if you’re used to subnetting to conserve IPv4 addresses, but you need to change your mindset when working with IPv6 and realize that there is absolutely no need to worry about wasting addresses.

Rather the concern is how many subnets fit within a certain network. If you’ve been assigned a /48, how many /64s can you place inside it? These are the questions that IPv6 subnetting asks. You can get try it out with our IPv6 subnetting practice.

IPv6 is written in hexadecimal which uses the digits 0-9 and a-f. A hexadecimal digit represents 4 bits, or 16 decimal units. Each IPv6 address is represented by eight groups of four hexadecimal digits. These groups are separated by colons. An example is: 2001:0db8:2231:aaec:0000:0000:4a4a:2100. You can use our Binary Decimal Hexadecimal Calculator to convert between the different values.

Even though IPv6 addresses are written in hexadecimal units, they are still large and cumbersome. There are a couple of ways to shorten them. One or more leading zeros from each group can be dropped. A consecutive set of groups with only zeros can be replaced with ::. Putting this into practice, the above address can be written as: 2001:db8:2231:aaec::4a4a:2100.

Note that the :: cannot be used on more than one section of zeros, and can only be used once per address. 2001::ab32:: is not a valid address because we do not know how many sets of zeros are in each :: group. The address could be 2001:0000:0000:ab32:0000:0000:0000:0000 or it could be 2001:0000:0000:0000:0000:ab32:0000:0000.

Correct IPv6 Notation

IPv6 addresses should be written a certain way. This will make interpreting the addresses faster and easier, both for humans and for computer programs.

Leading zeros should be removed. 2001:0db8::0001 should be written as 2001:db8::1.

“::” must be used to represent the largest number of 16-bit sets of zero as possible.

If there are multiple places where “::” can be used, and the numbers of zeros are the same, use “::” on the leftmost set of zeros.

“::” cannot be used to shorten a single 16-bit set of zeros.

Letters in an IPv6 address should be written in lowercase.

To represent port numbers, wrap an IPv6 address in square brackets then followed by a colon and the port number as [2001:db8::1]:80

For more information see RFC5952.

Though the representation of the numbers is different, the computer still uses binary to do the actual subnetting. The binary math is the same, it’s just using larger numbers. There is still a network portion and a host portion of every address.

IPv6 does not use broadcast addresses, however the same effect can be achieved by sending multicast to ‘all hosts’

We also do not use dotted decimal subnet masks for IPv6. Everything is written in slash notation. In IPv6 the first 48 bits are used for networking and routing. The next 16 are used to define subnets. The last 64 are used to identify hosts.

A feature of IPv6 is that every device can create a unique link local address based on the MAC address of the device. The way it works is by inserting 0xFFEE into the middle of the MAC address. This changes the MAC address to 64-bits from 48-bits. The other thing that needs to happen is to flip the 7th bit. A link local unicast address is fe80::/10

::1/128 is the loopback address.

::FFFF:0:0/96 are the IPv4-mapped addresses.

fe80::/10 are the link-local unicast addresses.

2001:db8::/32 are the documentation addresses.

ff00::/8 are multicast addresses.

Subnetting IPv6

The number of subnetting bits is the new prefix length minus the original prefix length. So there are 4 subnetting bits when a /48 is broken into /52s (52-48 = 4).

The number of subnets possible with n subnetting bits is 2n. So if we have 4 subnetting bits, then we can create 24 = 16 new subnets.

It’s common for an organization to be assigned a /48, which they can then allocate to their internal networks following the best practice of always using /64 for subnets with end hosts.

Each portion of an IPv6 address is 16 bits long, so a /48 falls on the boundary between the 3rd and 4th portions.

Let’s say the organisation has been assigned 2001:db8:1::/48 for our example.

They can now assign the 65536 available associated /64 subnets to their internal networks:
2001:db8:1:0::/64 2001:db8:1:1::/64 2001:db8:1:2::/64 2001:db8:1:3::/64 2001:db8:1:4::/64 Etc., all the way up to 2001:db8:ffff::/64

You can see that real world IPv6 subnetting is very easy and intuitive. When an organization is assigned a /48, all of the internal networks start with the same value for the first 3 portions of their IP addresses (2001:db8:1 in our example), and each internal subnet has a different value for the 4th portion (2001:db8:1:0, 2001:db8:1:1 etc in our example). It’s very simple to understand and work with.

You can use our IPv6 subnet calculator if you need to figure out a more complicated example.

Download our Subnet Cheat Sheet for all the essential information you need to quickly perform subnet calculations in your head.

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