IPv6 address types (and how to compress them)

IPv6 gives every interface several addresses at once, each with a scope. Knowing which range is which stops a lot of confusion when you read a routing table or write an ACL.

The ranges to know

• →2000::/3 - global unicast (globally routable).
• →fe80::/10 - link-local (valid only on the link; used by NDP and OSPFv3/routing adjacencies).
• →fc00::/7 (in practice fd00::/8) - unique local (private, like RFC1918).
• →ff00::/8 - multicast. ::1 - loopback.

Two rules to compress

• →Drop leading zeros within each 16-bit group.
• →Replace the single longest run of all-zero groups with :: (only once per address).

2001:0db8:0000:0000:0000:ff00:0042:8329 compresses to 2001:db8::ff00:42:8329.