Sometimes networking is cool, sometimes it’s just weird. Linux loves to use IPv4-mapped IPv6 addresses when passing information to servers, so here’s a quick cheat sheet on working them out…
| IPv4 | IPv6 Mapped |
|---|---|
127.0.0.1 | ::1 technically or ::ffff:127.0.0.1 |
10.0.0.1 | ::ffff:10.0.0.1/128 (or just ::ffff:10.0.0.1) |
10.0.0.0/24 | ::ffff:10.0.0.0/120 |
10.0.0.0/16 | ::ffff:10.0.0.0/112 (16 + 96 = 112) |
10.0.0.1/32 | ::ffff:10.0.0.1/128 (32 + 96 = 128) |
You can write the IPv4 part in hex too if you want to be extra annoying: ::ffff:a00:0/120 (since 10.0.0.0 = 0x0a000000).
The /120 is calculated by adding 96 to your IPv4 prefix length.
The formula: IPv6 prefix = IPv4 prefix + 96
Why 96? The ::ffff: prefix takes up 96 bits (the first 96 bits are fixed).