| IPv6 Network - Handling Multiple Addresses |
IPv6 Network - Handling Multiple Addresses - Even when IPv4-mapped addresses are supported, it isn’t always enough to replace the traditional socket API calls with the ones that support IPv6 to make an application completely IPv6-compatible. Some protocols embed IP addresses inside the protocol. For instance, peerto- peer applications need to tell one peer about another, and they nearly always use IP(v4) addresses for this. These protocols must be changed to support IPv6, as do the applications that implement them. The tragedy here is that applications that don’t need any changes, except using the new API, are exactly those that work well with Network Address Translation (NAT) in the first place, and one of the big advantages of IPv6 is that it eliminates the need for NAT. So this lands us in the position where most of the applications that support IPv6 are the ones that need it the least. A good example is the Web
Because an IPv6 system with a single IPv6 address more often than not also has an IPv4 address, it’s a very good idea for IPv6 applications to cycle through all the addresses that they find in the DNS when looking up the name for a remote system. This way, when the first (IPv6) address doesn’t work, the application still gets to connect to a subsequent (IPv4) address.
Although it’s possible for a single host to have more than one address in IPv4, most IPv4 applications don’t try the extra addresses when connecting to the first one fails, so this functionality must be added when IPv6-enabling an application. Higher-level APIs can generally be modified to support IPv6 without breaking existing applications. For instance, the Java java.net package has an InetAddress class, which always represented an IPv4 address. When Sun added IPv6 support in the J2SDK/JRE 1.4 release, the InetAddress class was summarily changed to encompass both IPv4 and IPv6 addresses, making all Java applications IPv6-capable in the process. That is, as long as both the Java Runtime Environment and the system support IPv6. This is a mixed blessing, because in addition to allowing a large number of Java programs to work over IPv6 without changes, it may also break some applications that make assumptions about the IP address length.
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