Ipv6 | XB Field Notes

An AAAA answer identifies an IPv6 destination; a separate connection verifies whether it works.

AAAA records publish IPv6 addresses without proving reachability

An AAAA resource record contains one 128-bit IPv6 address. An A record contains an IPv4 address. A name may have either type, both types, or several addresses within each type. These are separate RRsets, so A and AAAA can have different TTLs. Publishing AAAA makes an address discoverable through DNS; it does not create an interface, route, listener, or working application. ...

IPv6 fragments leave the original sender, while an intermediate router returns Packet Too Big for an oversized unfragmented packet.

IPv6 routers report oversized packets; sources decide how to fit the path

An IPv6 router does not fragment a packet while forwarding it toward its destination. If the packet is too large for the next link, the router normally discards it and sends ICMPv6 Packet Too Big feedback. The source can then adjust its packet size or, where appropriate, send fragments using the IPv6 Fragment header. Reassembly of those fragments takes place at the destination. ...

Several IPv6 spellings can identify the same 128 bits; a zone or port adds separate endpoint information.

IPv6 text normalization preserves addresses, but endpoint context still matters

An IPv6 address contains 128 bits, usually written as eight hexadecimal groups. Text may omit leading zeros and replace one consecutive run of zero groups with ::. These rules preserve the address while shortening its spelling. RFC 4291 defines valid forms; RFC 5952 recommends a consistent representation for display and exchange. Valid syntax and the preferred spelling are related but different questions. ...

Blocking IPv6 neighbor advertisements leaves the destination unresolved even when its TCP service is listening.

IPv6 resolves neighbors with ICMPv6 before the TCP connection can work

An IPv6 host on an Ethernet link needs a link-layer destination before it can send a frame to a neighbor. Neighbor Discovery performs that address-resolution work using ICMPv6 Neighbor Solicitation and Neighbor Advertisement messages. It also covers functions beyond IPv4 ARP, including router discovery and neighbor reachability detection. Calling it merely “ARP with longer addresses” hides dependencies that matter when configuring an IPv6 network. ...

The first address can remain preferred even after its service stops; client connection handling chooses the successful alternative.

Dual-stack address ordering and successful connection choice are separate

Discovering A and AAAA records provides possible destinations. It does not select a successful connection by itself. Address ordering, source-address selection, routing, and the application’s connection algorithm all contribute. RFC 6724 describes default source and destination selection rules, while applications can impose their own choices. A program requesting only IPv4 has already narrowed the result before any dual-stack connection strategy can act. ...