Introduction to IPv6 · Topic 1 of 3

IPv4 versus IPv6 and Address Notation

Compare IPv4 and IPv6, see how IPv6 address space is handed out from IANA to end sites, and learn the rules for writing and shortening IPv6 addresses.

  • 4 min
In a nutshell
  • IPv6 addresses are 128 bits, written as eight groups of four hex digits.
  • You may drop leading zeros in a group and use :: once for a run of zero groups.
  • IPv6 has no broadcast, and ARP is replaced by Neighbor Discovery.

What it is

IPv6 is the newer version of the Internet Protocol, built to fix the shortage of IPv4 addresses.

An IPv4 address has 32 bits, which gives about 4.3 billion addresses. An IPv6 address has 128 bits, which gives a number so large that every site can receive a huge block. The extra space also lets networks drop tricks such as NAT.

IPv6 is not just IPv4 with longer addresses. The packet header is simpler, broadcast is gone, and address setup is built into the protocol.

IPv432 bit addresstoo few addressesIPv6128 bit addressplentifulEvery sitegets a large block

How it works

IPv6 changes the address size, the header, and the way hosts find each other.

Main differences

FeatureIPv4IPv6
Address length32 bits128 bits
Written asDotted decimalHexadecimal groups separated by colons
BroadcastYesNo, multicast is used instead
Address resolutionARPNeighbor Discovery over ICMPv6
Header checksumYesNo
FragmentationSource and routersSource only
Address setupManual or DHCPManual, SLAAC or DHCPv6
Need for NATCommonNot needed for address shortage

The IPv6 header has a fixed size and no checksum, so routers do less work per packet. Options move into extension headers that most routers ignore. IPsec support is part of the protocol design, although using it is optional.

On RouterOS, IPv4 and IPv6 are configured in separate menus. You use /ip address for IPv4 and /ipv6 address for IPv6.

In RouterOS v6 the ipv6 package must be installed and enabled. In RouterOS v7 IPv6 is part of the main package.

How address space is distributed

IANA hands large blocks to the five Regional Internet Registries. The registries are AFRINIC, APNIC, ARIN, LACNIC and RIPE NCC.

A registry gives a block to a Local Internet Registry, usually an ISP. The ISP then gives each customer a prefix, often a /48 or /56 for a site. A single /64 is the usual size for one network segment.

IANAtop level poolRegional registryAPNIC or RIPE NCCISPlarge prefixCustomer sitesmaller prefixLANone /64

Writing IPv6 addresses

An address has eight groups of four hex digits. Each group is 16 bits.

2001:0db8:0000:0000:0000:0000:0000:0001

There are two shortening rules:

  • Drop leading zeros in each group, so 0db8 becomes db8 and 0000 becomes 0.
  • Replace one run of all zero groups with ::. You may use :: only once in an address.

Applying both rules gives the short form.

2001:db8::1

A prefix is written as an address, a slash and a length. For example 2001:db8:1::/48 is a 48 bit prefix. The slash length counts bits from the left, exactly like an IPv4 prefix length.

To expand ::, count the groups you can see and fill in zero groups until there are eight. The address fe80::1 shows two groups, so six zero groups hide inside the ::.

Exam tip

Hex digits are case insensitive. Lowercase is the usual way to write them, and RouterOS prints addresses in lowercase.

What to remember for the exam

Seven facts cover most questions on this topic.

  • IPv6 addresses are 128 bits, written as eight groups of four hex digits.
  • You may drop leading zeros in a group and use :: once for a run of zero groups.
  • IPv6 has no broadcast, and ARP is replaced by Neighbor Discovery.
  • The IPv6 header has a fixed size and no header checksum.
  • Routers do not fragment IPv6 packets. Only the source does.
  • A /64 is the normal prefix length for one LAN segment.
  • Address space flows from IANA to the registries, then to ISPs, then to customers.
Watch out

Using :: twice makes an address ambiguous, so it is invalid. Only one run of zero groups can be shortened.

Common mistakes

These four mistakes appear often.

Using :: twice. The address cannot be expanded to a single result, so it is rejected.

Removing trailing zeros. Only leading zeros in a group can go. The group 1200 is not the same as 12.

Expecting broadcast. IPv6 uses multicast for what broadcast did, such as finding all hosts on a link.

Mixing up the config menus. IPv4 addresses go in /ip address and IPv6 addresses go in /ipv6 address. A router can hold both at once.

Check yourself

Work through these three questions.

  • How many bits does an IPv6 address have, and how is it written?

    It has 128 bits, written as eight groups of four hex digits separated by colons.

  • Which two rules shorten an IPv6 address?

    Drop leading zeros in each group, and replace one run of zero groups with ::, used only once.

  • Which protocol replaces ARP in IPv6?

    Neighbor Discovery, which runs over ICMPv6.

Topic 1 of 3 in Introduction to IPv6

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