July 23, 2026
IPv6 looking-glass habits if you still think in v4
I spent a decade treating IPv6 as a second tab I opened when a vendor made me. Then a customer’s dual-stack site broke only on v6, the v4 looking glass was spotless, and I lost an afternoon because I queried a /64 like it was a /24. It is not a /24.
If you still think in v4, you do not need a lecture about the size of the address space. You need a few habits so the glass stops lying to you.
Query the prefix you announced, not the one you assigned
On v4 I lazily punch in the host and let the covering prefix appear. On v6, a host in 2001:db8:1:2::10 might live in a /48 you announced and a /56 you delegated and a /64 on the LAN. The RIB cares about what is in BGP. Start from the announced prefix. If you do not know it, look up the origin ASN’s v6 set in ASN explorer before you invent a CIDR.
A lot of “missing IPv6 route” tickets are people querying a /128 or a LAN /64 that was never meant to be global. The route is fine. The question was wrong.
IPv6 ping is not optional
v6 can be announced and still blackholed by a forgotten ACL, a RA guard fantasy, or a hypervisor that never got a default. An IPv6 ping from probes in more than one region tells you if the prefix answers before you start rewriting IS-IS. If v4 pings and v6 does not, do not debug BGP first. Debug forwarding and filters first. I say this as someone who has debug-BGP-first’d it more than once.
ICMPv6 is not the enemy. If you rate-limit it to death you will also rate-limit your own PMTUD and then wonder why people on weird tunnels cannot load your site. That ticket will not say “you murdered ICMPv6.” It will say “the site is slow on mobile.”
RPKI and IRR exist on v6 too
People sign their v4 ROAs and leave v6 unknown. Hijackers can read a calendar. Sign the v6 aggregates you announce. Check them with the same RPKI tool you use for v4. IRR route6 objects still matter for networks that filter on IRR; a missing route6 is a missing session in someone else’s prefix-list, and they will not page you. Your prefix will just be dark in their table.
Looking-glass muscle memory
Always say the family out loud. I have pasted a v6 address into a v4-only box and stared at a parser error like it had insulted me. Use a glass that takes both. Compare collectors the same way you would for a hijack. v6 tables are sparser in some regions; a “missing” path might be a collector that simply does not carry much v6. Get a second view before you accuse a peer.
And stop calling /48s “big.” A /48 is a customer assignment. A /32 is what you probably got from the RIR. If you announce random /64s into the DFZ, some of us will filter you, and we will be right.
Think in announced prefixes, ping both families, sign both families. That is the whole v6 conversion kit for people who still dream in dotted quad.