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IPv6 Addressing

IPv6 address types:

  • unicast - 1 to 1
  • multicast - 1 to all
  • anycast - 1-1 of many (special unicast)

Allows for rough load balancing at the IP layer. Works by advertising the same CIDR from two different places. Allows traffic to be routed to nearest service endpoint based on BGP advertised routes instead of a solution higher in the system stack, such as GeoIP/GeoList, AWS Global Accelerator, or load balancing manually.

Anycast Motivation snippet1:

A host transmits a datagram to an anycast address and the internetwork is responsible for providing best effort delivery of the datagram to at least one, and preferably only one, of the servers that accept datagrams for the anycast address.

Two types:

  • Within a network: reserved subnet anycast address (RFC 2526)
  • Between two networks: shared unicast address (requires application support? BGP?)

7 bit anycast ID

Limited by route sharing mechanism, so (externally) the most specific route entry is a /48. (BGP)

have to devote an entire allocation, makes entire range anycast.

Internally, could advertise more specific routes, so only a /56 or /64 is anycast.


Scenario base image


  • unicast region a - 2001:db8:6000::/48
    • Services present and accessible in region a
  • unicast region b - 2001:db8:6001::/48
    • Services present and accessible in region b
  • unicast region c - 2001:db8:6002::/48
    • Services present and accessible in region c
  • anycast a - 2001:db8:7000::/48
    • Services duplicated across all regions (stateless)
  • anycast b - 2001:db8:8000::/48
    • Services present in 1 region, but accessible from all


Other references


BGP tools:



  • Vultr communities guide:
  • Cloudlfare communities:
  1. RFC 1546 - Host Anycasting Service, 1993