1. IPv4 and IPv6 Interoperability
IPv4 client, IPv6 server over dual-stack server host
IPv6 client over dual-stack client host, IPv4 server
IPv6 address macro, function and option
Source code portability

2. over Dual-Stack Server Host
IPv4 Client, IPv6 Server
over Dual-Stack Server Host
IPv6
client
TCP
data
link
IPv4
client
TCP
data
link
IPv6
server
TCP
IPv4 IPv6
data
link
IPv6 listening
socket bounded
to 0::0, port 8888
IPv4-mapped
IPv6 address
IPv6 address
5f1b:df00:ce3e:e200:
20:800:2b37:6426
Ether IPv4 TCP TCP
hdr hdr hdr data
type dport
Ether IPv6 TCP TCP
hdr hdr hdr data
type dport
86dd

3. Dual-Stack Kernel at Server Side
AF_INET
SOCK_STREAM
sockaddr_in
AF_INET
SOCK_DGRAM
sockaddr_in
IPv4 sockets
AF_INET6
SOCK_STREAM
sockaddr_in6
AF_INET6
SOCK_DGRAM
sockaddr_in6
IPv6 sockets
TCP UDP
IPv4 IPv6
IPv6
IPv4
address
returned by
accept or
recvfrom
IPv4-
mapped
IPv4-
mapped
IPv4
IPv6
IPv4 datagram
IPv6 datagram

4. Dual-Stack Kernel at Client Side
AF_INET
SOCK_STREAM
sockaddr_in
AF_INET
SOCK_DGRAM
sockaddr_in
IPv4 sockets
AF_INET6
SOCK_STREAM
sockaddr_in6
AF_INET6
SOCK_DGRAM
sockaddr_in6
IPv6 sockets
TCP UDP
IPv4 IPv6
IPv6
IPv4
address for
connect or
sendto
IPv4-
mapped
IPv4-
mapped
IPv4
IPv6
IPv4 datagram
IPv6 datagram

5. Interoperability between IPv4 and IPv6
Clients and Servers
IPv4 server IPv6 server IPv4 server IPv6 server
IPv4-only host IPv6-only host dual-stack host dual-stack host
(A only) (AAAA only) (A and AAAA) (A and AAAA)
IPv4 client, IPv4 (no) IPv4 IPv4
IPv4-only host
IPv6 client, (no) IPv6 (no) IPv6
IPv6-only host
dual-stack host
IPv6 client, IPv4 IPv6 (no*) IPv6
*: IPv4-mapped IPv6 address if client chooses A record; no if client chooses AAAA record
Note that most implementations of IPv6 client/server will be on dual-stack hosts. (Second
row and second column may disappear.)

6. IPv6 Address Macro, Function, Option
IPv6 address testing macros:
IN6_IS_ADDR_* (e.g. V4MAPPED)
Protocol independent socket address functions:
sock_* (e.g. cmp_addr) (section 3.8)
IPv6_ADDRFORM socket option:
change a socket type between IPv4 and IPv6, by setsockopt function with IPv6_ADDRFORM option

7. Source Code Portability
Automatic program conversion from IPv4 to IPv6 (sockaddr_in, AF_INET, etc) and #ifdef to use IPv6 when possible
Deal with socket address structures as opaque objects:
remove gethostbyname and gethostbyaddr, use getaddrinfo and getnameinfo (which use #ifdef internally)