* can theoretically exist, but don't show up in the wild Once broken down like this, we can see that cone-ness isn’t terribly useful to us. The major distinction we care about is Symmetric versus anything else — in other words, we care about whether a NAT device is EIM or EDM. While it’s neat to know exactly how your firewall behaves, we don’t care from the point of view of writing NAT traversal c
![How NAT traversal works](https://cdn-ak-scissors.b.st-hatena.com/image/square/234c3a1f28208a7da8ae0cc88d8c745cfe09bc77/height=288;version=1;width=512/https%3A%2F%2Fcdn.sanity.io%2Fimages%2Fw77i7m8x%2Fproduction%2Fbbf71d57c972243cea664219fc80dcfc301252c6-1200x600.png)