1

I would like to know:

1) What is the smallest routable subnet size in IPv6 ?

2) Can a /128 address be routed in a IPv6 network?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 15 '17 at 4:44
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Routing works the same in IPv6 as IPv4, so yes, a /128 address can be routed.
However, you should not create subnets with prefixes longer than /64**.

RFC 7421 explains why.

** The exception is point to point links using a /127, as described in RFC 6164

  • 2
    And don't expect anything smaller than a /48 to be routed globally. – Sander Steffann Oct 31 '16 at 17:33
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A IPv6 /128 is a host route, exactly like a IPv4 /32.

A /127 is a point-to-point route, exactly like a /31. In the IP allocation database we'd usually allocate a /127 as the only network in an unadvertised /124 block, for textual representation reasons (point to point links then always end in 0 or 1, rather than requiring engineers to do binary math to work out which address goes on the interface nearer the core).

All other subnets must be /64. You must not use a /64 on a point-to-point link, as that allows a DoS.

As far as subnet address planning goes, subnet 0 is has a special textual representation and should be reserved for control-plane addresses of networking equipment. It is useful to reserve the lower 12 bits of the subnet addressing for a copy of the VLAN ID, as that gives a rapid and simple addressing plan for most subnets. The 13th bit obviously is 0 for VID-derived subnets and subnet 0; and 1 for others. Depending on your network you could use the highest subnetwork bits as a site identifier, allowing easy site address aggregation, and more effective area design (preventing flaps of non-backbone networks being propagated to other sites.).

(Not seeking to be an answer, my comment got too long, vote up Ron's answer.)

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