Definitive guide to understanding how to formulate an IPv6 address

Question

For fun, I\'d like to better understand the building blocks or elements that are within an IPv6 address.

Here are the basics, from my understanding:

• IPv6 a

Answer 1

IPv6 addresses with zone ID has a unique notation though its not specific to the address itself. In a system with multiple IPv6 enabled interfaces the link local address has to be disambiguated in some way. Its done using the "%" notation.

FE80::AA%eth0 refers to the link local address reachable through the eth0 interface. It can be an interface index in some plaforms, FE80::AA%10

Answer 2

Obviously this bit is a typo:

it is possible to use double colons syntax to represent one or more blocks with zeroes. 1:2::6:7 is equivalent to 1:2:3:4:5:6:7:8.

1:2::6:7 means 1:2:0:0:0:0:6:7.

I hadn't heard this before:

double colon may [not appear] within an ip4 dot address.

But I made a test program, and it seems to confirm it.

$ ./testipv6 0:0:0:0:0:0:192.168.0.1
0:0:0:0:0:0:192.168.0.1: OK

$ ./testipv6 0:0:0:0:0:0:192.168::1
0:0:0:0:0:0:192.168::1: ERROR

Otherwise I think everything you said is OK.

---

testipv6.c

#include <stdio.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <netinet/in.h>

int convert(const char *addr)
{
    struct in6_addr dst;

    return inet_pton(AF_INET6, addr, (void *)&dst);
}

int main(int argc, char **argv)
{
    if (argc == 1) {
        fprintf(stderr, "Usage: testipv6 <addr>\n");
        exit(2);
    }

    while (argc > 1) {
        argc--, argv++;
        const char *addr = argv[0];

        if (convert(addr)) {
            printf("%s: OK\n", addr);
        } else {
            printf("%s: ERROR\n", addr);
        }
    }
}

Answer 3

In general, yes, your points are correct.

Are you sure you read the RFC? RFC 3513, section 2.2 has exactly what you are asking for. It's very well written, for an RFC. =) I can't help but point this out since it may be very helpful to future people reading this question.