Calculating IPv6 Subnets in Linux

Jonathan MathewsPublic


We’re going to look at some IPv6 calculators, sipcalc and subnetcalc, and some tricks for subnetting without breaking our brains. Let’s start with reviewing IPv6 address types. There are three types: unicast, multicast, and anycast.

IPv6 Unicast

The unicast address is a single address identifying a single interface. In other words, what we usually think of as our host address. There are three types of unicast addresses:

  • Global unicast are unique publicly routable addresses. These are controlled by the Internet Assigned Numbers Authority (IANA), just like IPv4 addresses. These are the address blocks you get from your Internet service provider. These are in the 2000::/3 range, minus a few exceptions listed in the table at the above link.
  • Link-local addresses use the fe80::/10 address block and are similar to the private address classes in IPv4 (,, Some major differences are link-local addresses are not routable, but are confined to a single network segment. They are automatically derived from the MAC address of the network interface; this is not a guarantee that all of them are unique, but your odds are pretty good that they are. The IPv6 protocol requires that every network interface is automatically assigned a link-local address.
  • Special addresses are loopback addresses, IPv4-address mapped spaces, and 6-to-4 addresses for crossing from an IPv4 network to an IPv6 network.


Multicast in IPv6 is similar to the old IPv4 broadcast: a packet sent to a multicast address is delivered to every interface in a group. The IPv6 difference is that only hosts who are members of the multicast group receive the multicast packets, rather than all reachable hosts. IPv6 multicast is routable, and routers will not forward multicast packets unless there are members of the multicast groups to forward the packets to. Remember IPv4 broadcast storms? They’re much less likely to occur with IPv6. Multicast relies on UDP rather than TCP, so it is used for multimedia streaming, such as efficiently streaming the video feed from a single IP camera to multiple hosts. See IPv6 Multicast Address Space Registry for complete information.

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