Why is INET6_ADDRSTRLEN defined as 46 in C?

Question

The following program and its output shows that INET_ADDRSTRLEN is defined as 16 and INET6_ADDRSTRLEN is defined as 46.

Here is the program.

#include <stdio.h>
#include <arpa/inet.h>

int main()
{
    printf("%d\n", INET_ADDRSTRLEN);
    printf("%d\n", INET6_ADDRSTRLEN);
    return 0;
}

Here is the output.

16
46

I can understand why INET_ADDRSTRLEN needs to be 16. The largest possible string representation of an IPv4 address consumes 15 bytes, e.g. "255.255.255.255". Therefore 16 bytes are required to store such an IP address with its terminating null character.

But why does INET6_ADDRSTRLEN need to be 46? The largest possible string representation of an IPv6 address consumes only 39 bytes (according to my knowledge), e.g. "ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff". Therefore only 40 bytes are required to store such an IP address with its terminating null character.

Is there a string representation of an IPv6 address that can consume 46 bytes?

Answer 1

Why is INET6_ADDRSTRLEN defined as 46 in C?

Because POSIX defines it to be 46:

INET6_ADDRSTRLEN
46. Length of the string form for IPv6.

While you are right that longtest IPv6 address takes 39 bytes, with IPv4 tunneling, the longest form can be 45 bytes:

ffff:ffff:ffff:ffff:ffff:ffff:255.255.255.255

And the 46th byte is for the terminating nul byte (in C a string). This explains how it came to be 46.

Answer 2

It's probably for the IPv4-mapped form of addresses of the form:

ffff:ffff:ffff:ffff:ffff:ffff:255.255.255.255

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RFC 4291 section 2.2