At the WAN level, v6 is a lot like v4 in that they are both packet switched, using next hop routing and best effort delivery, except that in v6 the addresses are 128 bits, routers don't fragment, and any per-packet options have been punted to extension headers. Subnetting in v6 is similar to v4 CIDR: addresses chunk into an ISP network prefix, typically /13-/48; then around 16 bits of organizational subnetting; and last a host part, which is invariably 64-bits. In v6 there is a strong bias toward all subnets being /64 at the vlan except for point to point links, which are usually /127. Where v4 and v6 look a lot different is on the LAN side:
- v4 ARP gets replaced by v6 ICMP neighbor discovery
- no broadcast, but lots of multicast
- v6 makes heavy and ongoing use of fe80::/64 link-scope addresses, while v4 basically only exhibits 169.254.0.0/16 zeroconf addresses when it is broken
- v4 makes heavy use of RFC-1918 private addresses, usually in conjunction with NAT44. v6 has unique local addresses, but there is no NAT66, so they are rarely used.
- in v6, the gateway address is not a DHCP option, so the only way to dynamically find your upstream router is to scrape its link-local sender address off the ICMP router advertisement messages. While v4 in theory could have router advertisements, no one ever used them; in v6 they are required. The router advertisements also tell you which prefixes are on-link, and which of those, if any, are flagged as available for stateless address autoconfiguration.
- in v6, DHCP behavior is controlled by flag bits in the router advertisement; unlike v4 where it is the client's choice. There is a managed bit (DHCP vs SLAAC) for address choice, and an other config bit. So v6 can do "static DHCP", where the client autoconfigures a SLAAC address from a prefix in the RA, but then does DHCP for other network information like DNS servers and search domains, NTP servers, syslog servers, timezone offset, etc.
There is a bunch more introductory material from a half-day presentation I did a while back at:
https://mywebspace.wisc.edu/jeleinwe/web/ipv6/det/
-- Jim Leinweber, WI State Lab of Hygiene
