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<?xml version="1.0" encoding="UTF-8"?>
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<meta name="abstract" content="AF_INET6 sockets provide support for Internet Protocol version 6 (IPv6) 128 bit (16 byte) address structures. Programmers can write applications using AF_INET6 address family to accept client requests for either IPv4 or IPv6 nodes or from IPv6 nodes only." />
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<h1 class="topictitle1">Use AF_INET6 address family</h1>
<div><p>AF_INET6 sockets provide support for Internet Protocol version
6 (IPv6) 128 bit (16 byte) address structures. Programmers can write applications
using AF_INET6 address family to accept client requests for either IPv4 or
IPv6 nodes or from IPv6 nodes only.</p>
<div class="section"><p>Like AF_INET sockets, AF_INET6 sockets can be
either connection-oriented (type SOCK_STREAM) or they can be connectionless
(type SOCK_DGRAM). Connection-oriented AF_INET6 sockets use TCP as the transport
protocol. Connectionless AF_INET6 sockets use User Datagram Protocol (UDP)
as the transport protocol. When you create an AF_INET6 domain socket, you
specify AF_INET6 for the address family in the socket program. AF_INET6 sockets
can also use a type of SOCK_RAW. If this type is set, the application connects
directly to the IP layer and does not use either the TCP or UDP transport.</p>
</div>
<div class="section"><h4 class="sectiontitle">IPv6 applications compatibility with IPv4 applications</h4><p>Socket
applications written with AF_INET6 address family allow Internet Protocol
version 6 (IPv6) applications to work with Internet Protocol version 4 (IPv4)
applications (those applications that use AF_INET address family). This feature
allows socket programmers to use an IPv4-mapped IPv6 address format. This
address format represents the IPv4 address of an IPv4 node to be represented
as an IPv6 address. The IPv4 address is encoded into the low-order 32 bits
of the IPv6 address, and the high-order 96 bits hold the fixed prefix 0:0:0:0:0:FFFF.
For example, an IPv4- mapped address can look like this: </p>
<pre> ::FFFF:192.1.1.1</pre>
</div>
<div class="section"><p>These addresses can be generated automatically by the <span class="apiname">getaddrinfo()</span> function,
when the specified host has only IPv4 addresses. </p>
</div>
<div class="section"><p>You can create applications that use AF_INET6 sockets to open
TCP connections to IPv4 nodes. To accomplish this task, you can encode the
destination's IPv4 address as an IPv4–mapped IPv6 address and pass that address
within a sockaddr_in6 structure in the <span class="apiname">connect()</span> or <span class="apiname">sendto()</span> call.
When applications use AF_INET6 sockets to accept TCP connections from IPv4
nodes, or receive UDP packets from IPv4 nodes, the system returns the peer's
address to the application in the <span class="apiname">accept()</span>, <span class="apiname">recvfrom()</span>,
or <span class="apiname">getpeername()</span> calls using a sockaddr_in6 structure encoded
this way. </p>
</div>
<div class="section"><p>While the <span class="apiname">bind()</span> function allows applications
to select the source IP address of UDP packets and TCP connections, applications
often want the system to select the source address for them. Applications
use in6addr_any similarly to the way they use the INADDR_ANY macro in IPv4
for this purpose. An additional feature of binding in this way is that it
allows an AF_INET6 socket to communicate with both IPv4 and IPv6 nodes. For
example, an application issuing an <span class="apiname">accept()</span> on a listening
socket bound to in6addr_any accepts connections from either IPv4 or IPv6 nodes.
This behavior can be modified through the use of the IPPROTO_IPV6 level socket
option IPV6_V6ONLY. Few applications need to know which type of node with
which they are interoperating. However, for those applications that do need
to know, the IN6_IS_ADDR_V4MAPPED() macro defined in &lt;netinet/in.h&gt;
is provided.</p>
</div>
<div class="section"><h4 class="sectiontitle">IPv6 restrictions</h4><div class="p">Currently, i5/OS™ support
of IPv6 is limited in some functions. The following table lists
these restrictions and their implications to socket programmers.
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="border" border="1" rules="all"><caption>Table 1. IPv6 restrictions and implications</caption><thead align="left"><tr><th valign="top" id="d0e77">Restriction</th>
<th valign="top" id="d0e79">Implication</th>
</tr>
</thead>
<tbody><tr><td valign="top" headers="d0e77 ">iSeries™ host table does not support IPv6 addresses.</td>
<td valign="top" headers="d0e79 ">The <span class="apiname">getaddrinfo()</span> and <span class="apiname">getnameinfo()</span> APIs
are not able to locate IPv6 addresses in the host table. These APIs find addresses
in the Domain Name System (DNS) only.</td>
</tr>
<tr><td valign="top" headers="d0e77 ">The <span class="apiname">gethostbyname()</span> and <span class="apiname">gethostbyaddr()</span> APIs
only support IPv4 address resolution.</td>
<td valign="top" headers="d0e79 ">Use <span class="apiname">getaddrinfo()</span> and <span class="apiname">getnameinfo()</span> APIs
if IPv6 address resolution is required.</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
</div>
<div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="uaddrfam.htm" title="This topic includes sample programs that illustrate each of the socket address families.">Design applications with address families</a></div>
</div>
<div class="relref"><strong>Related reference</strong><br />
<div><a href="prerequisites.htm" title="Before writing socket applications, you must first complete these steps.">Prerequisites for socket programming</a></div>
<div><a href="ip6scen.htm" title="This topic describes a typical situation in which you might want to use the AF_INET6 address family.">Socket scenario: Create an application to accept IPv4 and IPv6 clients</a></div>
</div>
<div class="relinfo"><strong>Related information</strong><br />
<div><a href="../rzai2/rzai2compipv4ipv6.htm">Compare IPv4 to IPv6</a></div>
<div><a href="../apis/recvfr.htm">recvfrom()</a></div>
<div><a href="../apis/accept.htm">accept()</a></div>
<div><a href="../apis/gpeern.htm">getpeername()</a></div>
<div><a href="../apis/sendto.htm">sendto()</a></div>
<div><a href="../apis/connec.htm">connect()</a></div>
<div><a href="../apis/bind.htm">bind()</a></div>
<div><a href="../apis/ghostnm.htm">gethostbyname()</a></div>
<div><a href="../apis/getaddrinfo.htm">getaddrinfo()</a></div>
<div><a href="../apis/ghosta.htm">gethostbyaddr()</a></div>
<div><a href="../apis/getnameinfo.htm">getnameinfo()</a></div>
</div>
</div>
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